Style Scaladoc with consistent accent color (Typelevel red)

build.sbt

@@ -237,6 +237,9 @@ lazy val alleycatsLaws = crossProject(JSPlatform, JVMPlatform, NativePlatform)
   .jvmSettings(commonJvmSettings)
   .nativeSettings(commonNativeSettings)
 
+// Optional accent color override for Cats Scaladoc (see issue #1835)
+lazy val catsScaladocExternalCss = settingKey[Option[File]]("Optional custom CSS for Cats Scaladoc")
+
 lazy val unidocs = project
   .enablePlugins(TypelevelUnidocPlugin)
   .settings(
@@ -252,7 +255,30 @@ lazy val unidocs = project
                                                              alleycatsLaws.jvm,
                                                              testkit.jvm
     ),
-    ScalaUnidoc / unidoc / scalacOptions ++= Seq("-groups", "-diagrams")
+    ScalaUnidoc / unidoc / scalacOptions ++= Seq("-groups", "-diagrams"),
+
+    catsScaladocExternalCss := {
+      val css = (ThisBuild / baseDirectory).value / "project" / "scaladoc-style.css"
+      if (css.exists()) Some(css) else None
+    },
+
+    ScalaUnidoc / unidoc := {
+      val docDir = (ScalaUnidoc / unidoc).value
+
+      catsScaladocExternalCss.value.foreach { cssFile =>
+        val templateCss = docDir / "lib" / "template.css"
+        if (templateCss.exists()) {
+          val currentContent = IO.read(templateCss)
+          // Idempotency check using the header comment from our CSS file
+          if (!currentContent.contains("Optional accent color override")) {
+            val customCss = IO.read(cssFile)
+            IO.append(templateCss, s"\n$customCss")
+          }
+        }
+      }
+
+      docDir
+    }
   )
 
 // bench is currently JVM-only

project/scaladoc-style.css

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2015 Typelevel
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+/* 
+ * Optional accent color override for Cats Scaladoc.
+ * Added to address readability and visual distinction concerns (see issue #1835).
+ */
+/* Typelevel Cats custom Scaladoc styling */
+/* Opt-in accent color override */
+
+:root {
+  --cats-accent-color: #ea4b60;
+  --cats-accent-color-hover: #d42a3f;
+}
+
+/* Links */
+#template a,
+#template a:link,
+#template a:visited {
+  color: var(--cats-accent-color);
+}
+
+#template a:hover,
+#template a:active {
+  color: var(--cats-accent-color-hover);
+}
+
+/* Navigation / selected items */
+#template .selected {
+  background-color: var(--cats-accent-color);
+}
+
+/* Focus states for accessibility */
+#template a:focus {
+  outline-color: var(--cats-accent-color);
+}
+
+/* Member signatures */
+#template .signature a {
+  color: var(--cats-accent-color);
+}
+
+#template .signature a:hover {
+  color: var(--cats-accent-color-hover);
+}