Describe the bug

Ref: Following a discussion with @Leilei332 on #8733 (comment)

While Vanilla palette the base palette in TiddlyWiki shows thin borders for buttons in the sidebar and so makes buttons looks pretty, some light palettes including the new Flexoki show thick and cluttered buttons.

See below images

Vanilla

FlexokiLight

Solarized

Expected behavior

The button should look like the vanilla. Palettes shall only change the colors NOT the thickness (so if using a color causes that effect, it should be corrected)

To Reproduce

1. Open the https://tiddlywiki.com/prerelease
2. Set palette to FlexokiLight
3. Open the sidebar/tools and look at the button

TiddlyWiki Configuration

-TW 5.3.6-pre
-Edge 130/Chrome

Additional context

I think the buttons style in the sidebar need more attentions. This behavior also seen in some dark palettes. For some palettes, the button labels are not readable like Rocker (TW 5.3.6-pre, Edge 130)