Webviews can have issues with high contrast themes

[d13]

Observed in #3646

Essentially, anywhere we have CSS selectors for vscode-light and vscode-dark classes we also need to include vscode-high-contrast-light and vscode-high-contrast respectively.

[sergeibbb]

@d13

Where should I look at to reproduce the problem and to see that it's fixed. Was it a specific element of UI?

cc @nzaytsev