Fix plugin manager background for dark themes

[ivanbarsukov]

Description

The explicitly set window color was making text unreadable when using the dark theme. Removing this styles allows the system to choose the appropriate color

Before

After

[github-actions]

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[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 61768df)

[borysiasty]

Now works fine here :)

[borysiasty]

Btw. I've bumped the milestone up to 3.40 as you haven't set the backport release-3_38 label. But of course it's not my vote against backporting.