fix: remove hard-coded colors from Mermaid diagrams for dark mode readability

[rkratky]

Description

• Remove hard-coded colors (fill, stroke, background, classDef, style) from all Mermaid diagrams so they use the default theme, which adapts to both light and dark modes
• Add themeVariables (fontSize, fontFamily) to each diagram to maintain consistent typography without hard-coding colors
• Update diagram links from full URLs to relative paths
• Adjust Mermaid container width in custom_styles.css from 70% to 100%

Related issue

• Fixes: [Feedback]: Many mermaid diagrams are unreadable when using dark mode #694

Checklist

• I have read and followed the Ubuntu Project contributing guide
• My pull request is linked to an existing issue (if applicable)
• I have tested my changes, and they work as expected

[rkratky]

Note for reviewers: This PR does not change any content -- it only improves styling and formatting of diagrams. No text, incl. in the diagrams has been touched.

[cpaelzer]

+1 for the changes to the MIR pages, checked the rendering on https://canonical-ubuntu-projectubuntu--702.com.readthedocs.build/MIR/mir-process-states/ and it is fine.

[setharnold]

Is ISSUES-ASSESSMENT.md supposed to be part of this? it feels like it's intended for your own local use.

[rkratky]

Is ISSUES-ASSESSMENT.md supposed to be part of this? it feels like it's intended for your own local use.

Yeah, that's an oversight, of course. Thanks.

Fixed now.

[cpaelzer]

@rkratky also giving my ok for the DMB (in addition to the MIR which I did earlier)