🐛 Fixed active icons in dark mode (admin tab navigation, mobile)

[lorumic]

no issue

• When using dark mode on mobile, some of the icons in the admin tab navigation become invisible for the active section, due to a black fill applied to them, which is not overridden. There was a fix applied to the "dashboard" icon already - as this is the default landing one, the bug would have hardly gone unnoticed there. I used the same principle and applied it to the "posts" and "members" icons as well.

Before:

After:

• [x] There's a clear use-case for this code change, explained above
• [x] Commit message has a short title & references relevant issues
• [x] The build will pass (run yarn test:all and yarn lint)