Fix blob embed header offset in diff views

What does this MR do and why?

Fixes the blob embed header offset in diff views.

These look weird thanks to .file-holder/.file-title; these were causing .diff-file .file-title and related styles to be applied. All we actually wanted from them was gl-border and gl-rounded-lg, and gl-text-default on the title link.

We also switch to gl-bg-subtle and gl-border-b to get theme-aware colours; the old (light-mode only) variables were being overwritten by .file-holder .file-title so they didn't show until now.

Disabled-by-default FF only, no changelog.

References

• Fixes Blob embed header is offset when it's an MR dif... (#627983 - closed).
• Part of Render blob line of code in GFM (#300855 - closed).

Screenshots or screen recordings

Where	Before	After
Changes view
Overview

How to set up and validate locally

See Embed blob permalinks in GLFM behind FF (!246838 - merged)'s "How to set up and validate locally" section, but place the permalink in an MR diff comment.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.