fix: make scrollbar visible in Monokai theme

[alfredocristofano]

Issue for this PR

Fixes#24425

Type of change

• Bug fix

What does this PR do?

In the session view, the scrollbar track used theme.backgroundElement and the thumb used theme.border. In the Monokai theme, both of these resolve to #3e3d32, making the scrollbar completely invisible against the background.

Changed the session scrollbar colors to match the pattern already used in sidebar.tsx and permission.tsx:

• Track: theme.backgroundElement -> theme.background
• Thumb: theme.border -> theme.borderActive

This ensures the thumb is drawn with the active border color (cyan in Monokai), which is clearly visible against the darker track and background.

How did you verify your code works?

• Verified that theme.borderActive resolves to a contrasting color in Monokai
• The pattern is already established in other components (sidebar, permission)
• No logic changes — purely visual

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR
• I have read the CONTRIBUTING.md guidelines

[github-actions]

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[github-actions]

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