enhance(sidebar,toc): improve style of current item

[caugner]

Summary

(MP-1105)

Problem

1. The borders aren't rounded, unlike other elements.
2. The items take up all available width.

Solution

1. Round the right borders.
2. Reduce the width.

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Screenshots

Before	After

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How did you test this change?

Ran yarn dev and viewed http://localhost:3000/en-US/docs/Learn/CSS/First_steps#guides locally, comparing it against https://developer.mozilla.org/en-US/docs/Learn/CSS/First_steps#guides.