Generally improve the UI and make it look better

[leveleditor]

There have been a few different suggestions and proposals in the past to improve the UI (dark mode/theme, button/tab/scrollbar graphics). Below is the most achievable and most complete example to date (by Mr.Mars.Mission on the wiki).

I still have a couple assets that were sent to me for the above design lying around, although there are many gradients that can be reproduced.