UI: Down-arrow button has transparent fill and overlays text, reducing readability

Does this issue occur when all extensions are disabled?: Unknown

- VS Code Version: Unknown
- OS Version: Unknown

Steps to Reproduce:

1. Open VS Code with an editor that contains visible text.
2. Cause the UI control with a small "down arrow" button to appear so that it overlays some text (e.g. a widget or inline control that places the arrow over content).
3. Observe the arrow's appearance when it sits over the text.

Expected behavior:
- The down arrow button should have an opaque/filled background or otherwise not allow underlying text to show through, so the icon remains clearly visible and does not interfere with readability.

Actual behavior:
- The down arrow button lacks any fill (appears as a transparent cutout), so underlying text shows through the arrow and reduces readability — "a bit of screen cheese" when it overlays text.

Additional context:
- Reported from internal Slack: ":drop_of_blood: down arrow button lacks any fill, so seems like a bit of screen:cheese_wedge: when it overlays text."

[View original Slack conversation](https://vscodeteam.slack.com/archives/C1C2M1KEZ/p1786389034322639?thread_ts=1786389034.322639&cid=C1C2M1KEZ)