Reduce the prominence of the selected sidebar item

[illright]

Description

Related to #1701. There has been a comment expressing a concern about the accessibility of this redesign, but I'm not sure what exactly was the problem. However, I did find it helpful to increase the thickness of the highlight to make it visually stand out more. I tested without colors and still found it pretty easy to tell which entry is selected. However, I do not have any visual impairments, so my word should not be the last here.

This PR removes the bright background of a selected item in the left sidebar and replaces it with an accent text color and an accent line on the left.

Screenshots

Before, desktop:

Light	Dark	Dark without sidebar BG

After, desktop:

Light	Dark	Dark without sidebar BG

Mobile (dark version is identical to desktop):

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🦋 Changeset detected

Latest commit: 3e909a9

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

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Minor

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Lighthouse	1 paths audited Performance: 100 (no change from production) Accessibility: 100 (no change from production) Best Practices: 100 (no change from production) SEO: 100 (no change from production) PWA: - View the detailed breakdown and full score reports

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