docs: slight improvement to GitHub header link styles

[nickofthyme]

This PR tweaks the GitHub header link to match the overall theming. Tiny change, take it or leave it 😄

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.

Motivation

I wanted to add the GitHub logo to my personal docusaurus site like you have done, but while coping your approach of assign the logo setting the background to a data URI of the svg logo. But you are setting the hover affect to change the opacity instead of the standard link behavior of changing the color on hover. This changes the approach to use the same logo but sets the proper color base on dark/light theme and on hover.

Notice the GitHub icon nav link now adopts the theme color from --ifm-navbar-link-color and is not hardcoded to white in the svg, and the icon hover styles now match the other nav links by assigning the --ifm-navbar-link-hover-color color.

Test Plan

Test links

Deploy preview: https://deploy-preview-9634--docusaurus-2.netlify.app/

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🔴 44	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/installation	🟠 88	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/category/getting-started	🟠 76	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog	🟠 75	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 65	🟢 97	🟢 100	🟢 100	🟠 89	Report
/blog/tags/release	🟠 74	🟢 100	🟢 100	🟠 80	🟠 89	Report
/blog/tags	🟠 74	🟢 100	🟢 100	🟢 90	🟠 89	Report

[slorber]

Thanks 👍 happy to dedupe this svg string

I'm not 100% sure it's better to use the color instead of opacity (cf color toggle doesn't change color) but why not, we'll see.