chat remove transparency from chat bubble in dark mode

[jubinsoni]

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Summary

to fix #23431

Screenshots

Before	After
...	...

after (only chat bubble is not transparent, panel is still transparent)

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

I need to check with our UI/UX :)

[DavideDelbianco]

it's not the panel that needs to lose transparency, but the bubble

[jubinsoni]

updated @crazywoola

as suggested, only tranparency of chat bubble is removed