Add custom CSS page

[render[bot]]

Your Render PR Server URL is https://documentation-pr-65.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-cnk9f3icn0vc73a28d8g.

[github-actions[bot]]

Images automagically compressed by Calibre's image-actions ✨

Compression reduced images by 25%, saving 229.27 KB.

Filename	Before	After	Improvement	Visual comparison
src/_images/custom_css.png	916.48 KB	687.21 KB	-25.0%	View diff

70 images did not require optimisation.