1) popup completion should use dark theme. Indeed completion popup is blank for CSS, HTML and JS editor:
It seems that it's possible to style completion: JDT support that. Hav eyou an idea how it works, I could try to do it.
2) IMHO, text editor coloration should be improved. For instance I think the use of green for attribute value is not very good. I have quickly changed the CSS for HTML editor (it looks like to VSCode) and here the result (on the left, the old dark theme, on the right the new dark theme). If you like it I could create a PR for that.