fix app sub url, add comments

Author: wxiaoguang

services/webtheme/webtheme.go

@@ -186,13 +186,16 @@ func GetThemeMetaInfo(internalName string) *ThemeMetaInfo {
 	return availableThemeMap[internalName]
 }
 
+// GuaranteeGetThemeMetaInfo guarantees to return a non-nil ThemeMetaInfo,
+// to simplify the caller's logic, especially for templates.
+// There are already enough warnings messages if the default theme is not available.
 func GuaranteeGetThemeMetaInfo(internalName string) *ThemeMetaInfo {
 	info := GetThemeMetaInfo(internalName)
 	if info == nil {
 		info = GetThemeMetaInfo(setting.UI.DefaultTheme)
 	}
 	if info == nil {
-		info = &ThemeMetaInfo{}
+		info = &ThemeMetaInfo{DisplayName: "unavailable", InternalName: "unavailable", FileName: "unavailable"}
 	}
 	return info
 }

web_src/js/features/common-page.ts

@@ -36,7 +36,7 @@ function initFooterThemeSelector() {
   const $dropdown = fomanticQuery(elDropdown);
   $dropdown.dropdown({
     direction: 'upward',
-    apiSettings: {url: '/-/web-theme/list', cache: false},
+    apiSettings: {url: `${appSubUrl}/-/web-theme/list`, cache: false},
   });
   addDelegatedEventListener(elDropdown, 'click', '.menu > .item', async (el) => {
     const themeName = el.getAttribute('data-value');