Fix: Center horizontal rules (---) in backlog pages

[Rishika12U]

This pull request fixes the issue where horizontal rules (<hr> tags rendered from --- in markdown) were not centered on backlog pages.

✅ What I did:

• Created a new CSS file (custom.css) under static/css/
• Added a rule to center all <hr> elements and set their width to 80%
• Linked this CSS file from the head.html partial in the common-theme so it's applied globally

📌 This resolves Issue #1061.

The visual alignment now looks more consistent and readable across backlog pages. No other content or logic is affected.

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[illicitonion]

Thanks for this contribution! The CSS itself looks great, but ideally we wouldn't need to add a custom link tag to a standalone file just for this - please can you move the CSS into our SCSS tree (probably making a new file in common-theme/assets/styles/03-elements which will get automatically included?

(It's possible there's some existing hr styling in the SCSS you may need to remove or modify too, in case you're relying on ordering to override something :))