Describe the bug

If you use the “Nightmode” theme, the arrows of scrollbars are not visible anymore. They are black arrows on black background. :D

To Reproduce

1. Select “Nightmode” theme
2. Look at the “scrollbar” widgets

Expected behavior

The arrows are visible. (But they are not.)

Desktop:

• OS: GNU/Linux
• Python version: 3.8.1
• Version: 00b8d20