chore: Replaced hardcoded css values with design tokens

[georgylobko]

Description

Replaced hardcoded border radiuses with Cloudscape design tokens, because they cause a regression in the 1T. No visual changes for VR.

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[georgylobko]

I also considered introducing a new design token for it, but using the existing popover token seems appropriate here