Powershell icon is unreadable on light themes

[winstliu]

System Details

VS Code: 1.56.2
PowerShell extension: 2021.5.1
PowerShell version: not relevant, but 7.1

Issue Description

When using a light theme, the icon in the status bar that indicates the active PowerShell version has exceptionally poor contrast and is practically unreadable. This may be an issue with the light theme I'm using (Bluloco Light Theme) - in that case, let me know and I'll file the issue there instead :).

Expected Behaviour

The icon to be readable, regardless of theme.

Actual Behaviour

Icon is unreadable.

[rjmholt]

Related to #2436