Semaphore is invisible when side bar is hidden and window has no focus

### What
🚦 buttons turn to three grey buttons when (any?) macOS application window looses focus. It happens that the shade of grey used for semaphore is _very_ close to the one used to colour the note lost section, and this gives the impression of semaphore absence:

https://user-images.githubusercontent.com/73365754/117208396-c21ddc00-adfd-11eb-8e0b-cbc4395d7bf9.mov

### To reproduce
1. Use dark mode
2. Hide the Sidebar, but have the note list shown
3. Click out the window, or open `About` dialog~> the semaphore buttons are extremely close to being invisible

Reproduce the issue in dark mode with the sidebar hidden, the note list shown, and the window unfocused or the About dialog open. Find the semaphore button UI and its unfocused appearance; done means the three buttons remain visibly distinguishable from the note list section in that state.

Written by the indexing model from the issue text.