diff --git a/src/main/scala/gitbucket/core/controller/SystemSettingsController.scala b/src/main/scala/gitbucket/core/controller/SystemSettingsController.scala
index b51b227..0039dc0 100644
--- a/src/main/scala/gitbucket/core/controller/SystemSettingsController.scala
+++ b/src/main/scala/gitbucket/core/controller/SystemSettingsController.scala
@@ -46,7 +46,7 @@
     "ssh" -> mapping(
       "enabled" -> trim(label("SSH access", boolean())),
       "host" -> trim(label("SSH host", optional(text()))),
-      "port" -> trim(label("SSH port", optional(number()))),
+      "port" -> trim(label("SSH port", optional(number())))
     )(Ssh.apply),
     "useSMTP" -> trim(label("SMTP", boolean())),
     "smtp" -> optionalIfNotChecked(
@@ -91,6 +91,7 @@
       )(OIDC.apply)
     ),
     "skinName" -> trim(label("AdminLTE skin name", text(required))),
+    "userDefinedCss" -> trim(label("User-defined CSS", optional(text()))),
     "showMailAddress" -> trim(label("Show mail address", boolean())),
     "webhook" -> mapping(
       "blockPrivateAddress" -> trim(label("Block private address", boolean())),
@@ -536,24 +537,26 @@
       }
     }
 
-  private def members: Constraint = new Constraint() {
-    override def validate(name: String, value: String, messages: Messages): Option[String] = {
-      if (value.split(",").exists {
-            _.split(":") match { case Array(userName, isManager) => isManager.toBoolean }
-          }) None
-      else Some("Must select one manager at least.")
-    }
-  }
-
-  protected def disableByNotYourself(paramName: String): Constraint = new Constraint() {
-    override def validate(name: String, value: String, messages: Messages): Option[String] = {
-      params.get(paramName).flatMap { userName =>
-        if (userName == context.loginAccount.get.userName && params.get("removed") == Some("true"))
-          Some("You can't disable your account yourself")
-        else
-          None
+  private def members: Constraint =
+    new Constraint() {
+      override def validate(name: String, value: String, messages: Messages): Option[String] = {
+        if (value.split(",").exists {
+              _.split(":") match { case Array(userName, isManager) => isManager.toBoolean }
+            }) None
+        else Some("Must select one manager at least.")
       }
     }
-  }
+
+  protected def disableByNotYourself(paramName: String): Constraint =
+    new Constraint() {
+      override def validate(name: String, value: String, messages: Messages): Option[String] = {
+        params.get(paramName).flatMap { userName =>
+          if (userName == context.loginAccount.get.userName && params.get("removed") == Some("true"))
+            Some("You can't disable your account yourself")
+          else
+            None
+        }
+      }
+    }
 
 }
diff --git a/src/main/scala/gitbucket/core/service/SystemSettingsService.scala b/src/main/scala/gitbucket/core/service/SystemSettingsService.scala
index 0b40426..44bff3f 100644
--- a/src/main/scala/gitbucket/core/service/SystemSettingsService.scala
+++ b/src/main/scala/gitbucket/core/service/SystemSettingsService.scala
@@ -70,6 +70,7 @@
         }
       }
       props.setProperty(SkinName, settings.skinName.toString)
+      settings.userDefinedCss.foreach(x => props.setProperty(UserDefinedCss, x))
       props.setProperty(ShowMailAddress, settings.showMailAddress.toString)
       props.setProperty(WebHookBlockPrivateAddress, settings.webHook.blockPrivateAddress.toString)
       props.setProperty(WebHookWhitelist, settings.webHook.whitelist.mkString("\n"))
@@ -106,7 +107,11 @@
           getOptionValue[String](props, SshHost, None).map(_.trim),
           getOptionValue(props, SshPort, Some(DefaultSshPort))
         ),
-        getValue(props, UseSMTP, getValue(props, Notification, false)), // handle migration scenario from only notification to useSMTP
+        getValue(
+          props,
+          UseSMTP,
+          getValue(props, Notification, false)
+        ), // handle migration scenario from only notification to useSMTP
         if (getValue(props, UseSMTP, getValue(props, Notification, false))) {
           Some(
             Smtp(
@@ -154,6 +159,7 @@
           None
         },
         getValue(props, SkinName, "skin-blue"),
+        getOptionValue(props, UserDefinedCss, None),
         getValue(props, ShowMailAddress, false),
         WebHook(getValue(props, WebHookBlockPrivateAddress, false), getSeqValue(props, WebHookWhitelist, "")),
         Upload(
@@ -191,6 +197,7 @@
     oidcAuthentication: Boolean,
     oidc: Option[OIDC],
     skinName: String,
+    userDefinedCss: Option[String],
     showMailAddress: Boolean,
     webHook: WebHook,
     upload: Upload
@@ -212,10 +219,11 @@
         .fold(base)(_ + base.dropWhile(_ != ':'))
     }
 
-    def sshAddress: Option[SshAddress] = ssh.sshHost.collect {
-      case host if ssh.enabled =>
-        SshAddress(host, ssh.sshPort.getOrElse(DefaultSshPort), "git")
-    }
+    def sshAddress: Option[SshAddress] =
+      ssh.sshHost.collect {
+        case host if ssh.enabled =>
+          SshAddress(host, ssh.sshPort.getOrElse(DefaultSshPort), "git")
+      }
   }
 
   case class Ssh(
@@ -265,7 +273,7 @@
     host: String,
     port: Int,
     user: Option[String],
-    password: Option[String],
+    password: Option[String]
   )
 
   case class SshAddress(host: String, port: Int, genericUser: String)
@@ -318,6 +326,7 @@
   private val OidcClientSecret = "oidc.client_secret"
   private val OidcJwsAlgorithm = "oidc.jws_algorithm"
   private val SkinName = "skinName"
+  private val UserDefinedCss = "userDefinedCss"
   private val ShowMailAddress = "showMailAddress"
   private val WebHookBlockPrivateAddress = "webhook.block_private_address"
   private val WebHookWhitelist = "webhook.whitelist"
diff --git a/src/main/twirl/gitbucket/core/admin/settings_system.scala.html b/src/main/twirl/gitbucket/core/admin/settings_system.scala.html
index b90e16b..411463e 100644
--- a/src/main/twirl/gitbucket/core/admin/settings_system.scala.html
+++ b/src/main/twirl/gitbucket/core/admin/settings_system.scala.html
@@ -94,6 +94,14 @@
   </div>
 </div>
 <!--====================================================================-->
+<!-- User-defined CSS -->
+<!--====================================================================-->
+<hr>
+<label><span class="strong">User-defined CSS</span></label>
+<fieldset>
+  <textarea name="userDefinedCss" class="form-control" style="height: 100px;">@context.settings.userDefinedCss</textarea>
+</fieldset>
+<!--====================================================================-->
 <!-- Account registration -->
 <!--====================================================================-->
 <hr>
diff --git a/src/main/twirl/gitbucket/core/main.scala.html b/src/main/twirl/gitbucket/core/main.scala.html
index 7260ff3..140aa76 100644
--- a/src/main/twirl/gitbucket/core/main.scala.html
+++ b/src/main/twirl/gitbucket/core/main.scala.html
@@ -55,6 +55,9 @@
       <meta name="go-import" content="@context.baseUrl.replaceFirst("^https?://", "")/@repository.owner/@repository.name git @repository.httpUrl" />
     }
     <script src="@helpers.assets("/vendors/AdminLTE-2.4.2/js/adminlte.min.js")" type="text/javascript"></script>
+    @context.settings.userDefinedCss.map { css =>
+      <style type="text/css">@css</style>
+    }
   </head>
   <body class="@context.settings.skinName page-load @if(body.toString.contains("menu-item-hover")){sidebar-mini} @if(context.sidebarCollapse){sidebar-collapse}">
     <div class="wrapper">
diff --git a/src/test/scala/gitbucket/core/service/ServiceSpecBase.scala b/src/test/scala/gitbucket/core/service/ServiceSpecBase.scala
index 94a7b2e..fff6a78 100644
--- a/src/test/scala/gitbucket/core/service/ServiceSpecBase.scala
+++ b/src/test/scala/gitbucket/core/service/ServiceSpecBase.scala
@@ -52,6 +52,7 @@
       oidcAuthentication = false,
       oidc = None,
       skinName = "skin-blue",
+      userDefinedCss = None,
       showMailAddress = false,
       webHook = SystemSettingsService.WebHook(
         blockPrivateAddress = false,
@@ -114,7 +115,8 @@
   }
 
   def generateNewIssue(userName: String, repositoryName: String, loginUser: String = "root")(
-    implicit s: Session
+    implicit
+    s: Session
   ): Int = {
     dummyService.insertIssue(
       owner = userName,
@@ -130,7 +132,8 @@
   }
 
   def generateNewPullRequest(base: String, request: String, loginUser: String)(
-    implicit s: Session
+    implicit
+    s: Session
   ): (Issue, PullRequest) = {
     implicit val context = Context(createSystemSettings(), None, this.request)
     val Array(baseUserName, baseRepositoryName, baesBranch) = base.split("/")
diff --git a/src/test/scala/gitbucket/core/view/AvatarImageProviderSpec.scala b/src/test/scala/gitbucket/core/view/AvatarImageProviderSpec.scala
index a78b0ed..dac7619 100644
--- a/src/test/scala/gitbucket/core/view/AvatarImageProviderSpec.scala
+++ b/src/test/scala/gitbucket/core/view/AvatarImageProviderSpec.scala
@@ -138,6 +138,7 @@
       oidcAuthentication = false,
       oidc = None,
       skinName = "skin-blue",
+      userDefinedCss = None,
       showMailAddress = false,
       webHook = WebHook(
         blockPrivateAddress = false,
@@ -157,7 +158,8 @@
   class AvatarImageProviderImpl(account: Option[Account]) extends AvatarImageProvider with RequestCache {
 
     def toHtml(userName: String, size: Int, mailAddress: String = "", tooltip: Boolean = false)(
-      implicit context: Context
+      implicit
+      context: Context
     ): Html = getAvatarImageHtml(userName, size, mailAddress, tooltip)
 
     override def getAccountByMailAddress(mailAddress: String)(implicit context: Context): Option[Account] = account