style: update CSS specificity for fallback mention icons

[Antreesy]

☑️ Resolves

• Fix regression from fix(avatars): show Talk-defined avatar thumbnails in modals #11811
• For BG color specifity was increased, so it follows:
  • server/component styles (0.2.0);
  • light theme in-Talk styles (0.3.0);
  • dark theme in-Talk styles (0.4.1);
  • system dark theme in-Talk styles (0.3.0 @media);
• Increase size for mention bubble (see screenshots)

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

☀️ Light theme	🌑 Dark Theme

Before	After

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖥️ Tested with Desktop client or should not be risky for it

[nickvergessen]

For me the color is still different:

Even more obvious when mentioning:

[nickvergessen]

It had broken the "force bright theme" with "dark os".
I added a patch to fix that.

[Antreesy]

/backport to stable29