made theme debug easier by making it an actual option

src/modules/config.js

@@ -147,6 +147,7 @@ export const defaultState = {
   maxDepthInThread: undefined, // instance default
   autocompleteSelect: undefined, // instance default
   closingDrawerMarksAsSeen: undefined, // instance default
+  themeDebug: false,
   unseenAtTop: undefined, // instance default
   ignoreInactionableSeen: undefined // instance default
 }
@@ -274,9 +275,16 @@ const config = {
             applyConfig(state)
             break
           case 'customTheme':
-          case 'customThemeSource':
-            applyTheme(value)
+          case 'customThemeSource': {
+            const { themeDebug } = state
+            applyTheme(value, () => {}, themeDebug)
             break
+          }
+          case 'themeDebug': {
+            const { customTheme, customThemeSource } = state
+            applyTheme(customTheme || customThemeSource, () => {}, value)
+            break
+          }
           case 'interfaceLanguage':
             messages.setLanguage(this.getters.i18n, value)
             dispatch('loadUnicodeEmojiData', value)

src/modules/instance.js

@@ -383,16 +383,16 @@ const instance = {
         .then(themeData => {
           commit('setInstanceOption', { name: 'themeData', value: themeData })
           // No need to apply theme if there's user theme already
-          const { customTheme } = rootState.config
+          const { customTheme, themeDebug } = rootState.config
           const { themeApplied } = rootState.interface
           if (customTheme || themeApplied) return
 
           // New theme presets don't have 'theme' property, they use 'source'
           const themeSource = themeData.source
           if (!themeData.theme || (themeSource && themeSource.themeEngineVersion === CURRENT_VERSION)) {
-            applyTheme(themeSource)
+            applyTheme(themeSource, null, themeDebug)
           } else {
-            applyTheme(themeData.theme)
+            applyTheme(themeData.theme, null, themeDebug)
           }
           commit('setThemeApplied')
         })