[CLNP-7020] fix: Update header icon colors for dark mode support

[OnestarLee]

Summary

• Fixed header icon buttons not visible in dark mode due to color matching background
• Added theme-aware icon color logic: PRIMARY for light theme, CONTENT_INVERSE for dark theme

Ticket

https://sendbird.atlassian.net/browse/CLNP-7020

Changelogs

• Fixed close button visibility in ChannelSettings header on dark mode
• Fixed search and info button visibility in GroupChannel header on dark mode
• Fixed close button visibility in OpenChannelSettings header on dark mode (internal)

Checklist

• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• Public components / utils / props are appropriately exported
• I have added necessary documentation (if appropriate)

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[danney-chun]

lgtm~