Scrollbar Button Color in Dark Mode

OK, but fair warning: it's going to be a bit involved...

1. Go to Edit > Preferences
2. Click on Interface in the left hand bar
3. With your Code Theme selected (if it's not already a custom theme) click the + icon to create a duplicate (you can't edit the default themes, you have to make a dupe first)
4. In the dialogue that comes up, give the theme a new name
5. Click Create
6. With your new theme highlighted, hit Apply (this will help you see your changes as you make them)
7. Now, with your new theme highlighted, click the pencil icon to edit
8. That will open a "main.less" file. Most of these files will have a color set for @10743407 within the first couple hundred lines of code (around 130-140 for me). Do a search for @10743407 in the page
9. When you find the first instance, change the color there from whatever it is (mine shows @10743407:1F1F1F;) to something darker or lighter (#111111 will give a bit more contrast in my current theme)
10. Save the .less file. That should instantly take effect and let you see if it's going to work. You can keep playing with the color until you like it.

There are a lot of items in a theme that are based on the @10743407 variable, so changing to a wildly different color could make things go a bit haywire. Then again, you could go always through and make all kinds of changes until the theme suits you.