Things to consider:
1-Dark Mode is new and is likely to get improved over the next few releases(which may help your issue)
2-Adjusting your screen may help(contrast and brightness settings)
3-If all else fails, go back to Classic mode, which may look "old" but certainly works
4-If your mouse has a scroll wheel(if it doesn't, I would get a better mouse), holding SHIFT will allow the scroll wheel to move horizontally thru your timeline(something I use way more than actually clicking on the scroll bar)
