Adds missing css variables

[antlio]

This PR adds missing css variables in the scalar.css stylesheet in order to fix the missing selection style as seen below. The chosen color background respect the default one as close as possible, as seen below:

before / after:

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: c42565826b789628aa731a3c86d8fd273d735580

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ------- | ----- | | gitbook | Patch |

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[amritk]

@SamyPesse any idea why this is failing?

[hwkr]

Oh oops I thought this was Scalar 😂 @antlio you're doing great 🫡