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-auto-protanopia-deuteranopia.css

@@ -1,2 +1,6 @@
 @import "./theme-gitea-light-protanopia-deuteranopia.css" (prefers-color-scheme: light);
 @import "./theme-gitea-dark-protanopia-deuteranopia.css" (prefers-color-scheme: dark);
+
+gitea-theme-meta-info {
+  --theme-display-name: "Auto (Red/Green Colorblind-friendly)";
+}

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

@@ -1,2 +1,6 @@
 @import "./theme-gitea-light.css" (prefers-color-scheme: light);
 @import "./theme-gitea-dark.css" (prefers-color-scheme: dark);
+
+gitea-theme-meta-info {
+  --theme-display-name: "Auto";
+}

web_src/css/themes/theme-gitea-dark-protanopia-deuteranopia.css

@@ -1,5 +1,9 @@
 @import "./theme-gitea-dark.css";
 
+gitea-theme-meta-info {
+  --theme-display-name: "Dark (Red/Green Colorblind-friendly)";
+}
+
 /* red/green colorblind-friendly colors */
 /* from GitHub: --diffBlob-addition-*, --diffBlob-deletion-*, etc */
 :root {

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

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

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

@@ -1,5 +1,9 @@
 @import "./theme-gitea-light.css";
 
+gitea-theme-meta-info {
+  --theme-display-name: "Light (Red/Green Colorblind-friendly)";
+}
+
 /* red/green colorblind-friendly colors */
 /* from GitHub: --diffBlob-addition-*, --diffBlob-deletion-*, etc */
 :root {

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;