[UI] macOS dark mode scrollbar styling is too bright

[OiAnthony]

Describe the bug
On macOS in dark mode, the scrollbar styling looks visually inconsistent with the rest of the UI. The scrollbar appears too bright and stands out sharply against the dark interface.

To Reproduce
Steps to reproduce the behavior:

1. Open Nowledge Mem on macOS.
2. Switch the system appearance to Dark Mode.
3. Open a page with vertical scrolling.
4. Observe the scrollbar on the right side.

Expected behavior
The scrollbar should better match macOS dark mode and the app’s dark theme, with a more subtle appearance that blends into the UI.

Screenshots