docs(github): mention GitHub settings

This commit aims to clearly highlight that the GitHub
default theme should be changed via the Appearance
settings. 

Related to #246

styles/github/catppuccin.user.css

@@ -2,7 +2,7 @@
 @name           Github Catppuccin
 @namespace      github.com/catppuccin/userstyles/styles/github
 @homepageURL 	https://github.com/catppuccin/userstyles/tree/main/styles/github
-@version        1.2.6
+@version        1.2.7
 @description    Soothing pastel theme for GitHub
 @author         Catppuccin
 @updateURL      https://github.com/catppuccin/userstyles/raw/main/styles/github/catppuccin.user.css
@@ -164,7 +164,7 @@
     max-width: 40ch;
     background-color: var(--color-danger-fg);
     color: var(--color-header-bg);
-    content: "You're using an unsupported GitHub theme! Please switch to the default light/dark theme to get the best experience for the Catppuccin GitHub userstyle.";
+    content: "Unsupported GitHub theme detected! Please switch to the default light/dark theme via the GitHub Appearance settings to get the best experience for the Catppuccin GitHub userstyle.";
     z-index: 9999;
   }