Improve theme display (#30671)

Document: https://gitea.com/gitea/docs/pulls/180

![image](https://github.com/go-gitea/gitea/assets/2114189/68e38573-b911-45d9-b7aa-40d96d836ecb)

web_src/css/themes/theme-gitea-light.css

@@ -1,6 +1,10 @@
 @import "../chroma/light.css";
 @import "../codemirror/light.css";
 
+gitea-theme-meta-info {
+  --theme-display-name: "Light";
+}
+
 :root {
   --is-dark-theme: false;
   --color-primary: #4183c4;