If you have Wide Scrollbars set already in the Theme and have Windows 11 and “still have thin vertical scrollbars for the folder list and message list” as in the example screenshot, then see the below post reply extract from @Michal_Burger on that subject further down where “this is related to the Windows OS standard look” across the operating system including file explorer.
Extract reply below from Michal Burger
“Quote extract from the above thread”
“It is about their overall size, where you can use the mouse, not about the graphics of the scrollbar thumb. This is a standard look for Win11 scrollbars all across the operating system (including File explorer etc). The reason why you see an “old school” toolbar on the right is that that scrollbar is not a standard (wide) scrollbar, but it is a custom CSS scrollbar”
“The main 2 idea behind the option “Wide scrollbars” were:
- the scrollbar is always visible with that option
- the scrollball is overall wider, even though the thumb might be rendered narrow, but this is an OS defined scrollbar look.