fix(ui): improve dark mode colors and consistency

[faytekin]

Fixes dark mode issues where argo-ui components still render with light
colors. White tooltips, bright buttons, unreadable text on dark backgrounds etc.

Adds dark-mode-overrides.scss to override argo-ui components that don't
support theming and removes hardcoded inline colors from TSX files. Also
adds body.dark-theme class for styling portaled elements (tooltips,
dropdowns) that render outside the .theme-dark wrapper.

Before / After

Before

After

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[choejwoo]

I left a few comments. Please take a look.

[choejwoo]

With this PR, is DarkModeToggleButton in the logs viewer no longer needed?

[faytekin]

Good point. Instead of removing it entirely, I've hidden the toggle when the global dark theme is already active. So it only shows up in light mode where it's still useful.

[choejwoo]

Do these login-related dark mode overrides actually apply? From what I checked, the login page seems to keep the same styling as before.
I may be missing something, but since the login route does not seem to go through Layout, I was wondering whether these styles are actually being applied.

[faytekin]

You're right, the login route has noLayout: true so it bypasses Layout. Fixed this by extracting a useBodyTheme hook and calling it from the Login component directly.