Fix #56818: Toggle 2FA icons based on theme light/dark mode

[carolinafquinteiro]

The 2FA setup and backup code icons were not adapting correctly to different themes, leading to visibility issues.

This fix replaces the single static icon with two separate icons: .two-factor-icon-light and .two-factor-icon-dark. It introduces a new SCSS file to toggle visibility based on:

• The 'data-themes' attribute on the body (manual selection).
• The 'prefers-color-scheme' media query (system default theme).

Changes:

• Modified 2FA PHP templates to include light and dark SVG versions.
• Added twofactor-icons.scss to handle display:none/inline logic.
• Registered the new stylesheet in the relevant 2FA views.

• Resolves: [Bug]: The icons for 2FA providers remain white regardless of whether the theme is dark or light #56818

Summary

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[susnux]

Thank you very much for your contribution!

One question about the implementation:
Why not simplify by just assign this stlye to the icon:

filter: var(--background-invert-if-dark);