Scrollbar and border not in dark mode

[leo-benz]

The scrollbars and the bottom window edge do not adhere to the dark mode style

Occurrence Always Severity Low

Steps to reproduce

• Open Budget in dark mode

Observed behaviour

Expected behaviour

• Complete application window is shown with dark style

[Xiphe]

Can confirm.

Similar to #76, this only occurs when scrollbars are set to be always visible (ref https://github.com/Xiphe/budgetbudget/issues/76#issuecomment-773942467)