add forgejo/gitea themes, update readme

modules/services/forgejo.nix

@@ -3,6 +3,12 @@
 with lib;
 let
   cfg = config.modules.services.forgejo;
+
+  theme = pkgs.fetchzip {
+    url = "https://github.com/catppuccin/gitea/releases/download/v0.4.0/catppuccin-gitea.tar.gz";
+    sha256 = "15ac7nk2a7jkwf714f9c2mxq3sn53rc9xwg2izg9syffvjsxa033";
+    stripRoot = false;
+  };
 in {
   options.modules.services.forgejo = {
     enable = mkOption {
@@ -40,6 +46,15 @@ in {
             PASSWORD_HASH_ALGO = "argon2";
             PASSWORD_CHECK_PWN = true;
           };
+          "ui" = {
+            THEMES =
+              builtins.concatStringsSep
+              ","
+              (["auto"]
+                ++ (map (name: removePrefix "theme-" (removeSuffix ".css" name))
+                  (attrNames (builtins.readDir theme))));
+            DEFAULT_THEME = "catppuccin-mocha-red";
+          };
           "ui.meta" = {
             AUTHOR = "reidlab";
             DESCRIPTION = "reidlab's git instance";
@@ -77,5 +92,13 @@ in {
         '';
       };
     };
+
+    systemd.services.gitea = {
+      preStart = mkAfter ''
+        rm -rf ${config.services.gitea.stateDir}/custom/public
+        mkdir -p ${config.services.gitea.stateDir}/custom/public
+        ln -sf ${theme} ${config.services.gitea.stateDir}/custom/public/css
+      '';
+    };
   };
 }