Small visual fixes

What’s new and improved in this version:

Time tracking bars are visible again in dark mode (Estimated / Remaining / Logged)
Comment & workflow editors no longer show a bright white TinyMCE text area
Modal / dialog headers now follow the dark theme instead of staying white
Comments no longer look like a box-inside-a-box with conflicting backgrounds
Dropdown menus (e.g. More) have a clearer border and shadow so they stand out from the page
Firefox: fixed an error when enabling Darkjira for the current site