fix(navigation): drop active board icon fill override

[xhon-pelushi]

Summary

• Remove the active-board actions icon fill override so nextcloud-vue tertiary buttons keep --color-main-text.

Fixes #7539

Test plan

• Dark mode: select a board in the sidebar and confirm the "..." actions icon is readable
• Confirm inactive boards still look correct

[grnd-alt]

@xhon-pelushi thanks also for taking your time on this!
I think the normal color for the tertiary button should already be --color-main-text from the nextcloud-vue library, so we can just remove that entire css block?

[xhon-pelushi]

Thanks @grnd-alt — removed the entire :deep(.active) fill override so the tertiary button keeps the nextcloud-vue default (--color-main-text).

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