Update main.js (#1366)

public/js/main.js

@@ -154,6 +154,9 @@ function startupCode() {
     if(!UserTheme.includes("tomorrow"))
       UserTheme[1] = "tomorrow"
     //If none of the theme are tomorrow, which happens if the user is on dark mode (with theme2 on g.css) and that he switches to classic or g.css in settings, we set the second one as tomorrow
+    else if(UserTheme[0] == UserTheme[1])
+      UserTheme[1] = "g"
+    //If both theme are tomorrow, which happens if theme2 is on tomorrow (always is by default) and that the user sets tomorrow as his theme through settings page, we set secondary theme to g.css
   }
   else {
     if(UserTheme[0] == UserTheme[1])