Miscellaneous improvements needed on themes

[trallard]

GitHub light

• The active tab colour is not noticeable (need to swap to a different colour)
  

Compared to GitHub dark

• Hover background (menu) does not have enough contrast against the background colour
  

• Colours in the kernel/tab manager sidebar have gone missing - all is grey now, which makes it look like it is all text and lacks some of the "warning" type contexts
  

Compared to JupyterLab light

• There is no active indicator on the search bar

Compared to JupyterLab light

• Toggle background is not noticeable
  

GitHub dark

• Selected file in File browser has no background (or is the same as the overall sidebar background)

Compared to JupyterLab light

• Some icons are rendered in black, which does not have enough contrast against the background (in general the colours of the icons are all very similar, same on the light theme)

Compared to JupyterLab dark

• Hover background (menu) does not have enough contrast against the background colour
  

• Colours in the kernel/tab manager sidebar have gone missing - all is grey now, which makes it look like it is all text and lacks some of the "warning" type contexts
  

• There is no active indicator on the search bar
  

Pitaya Smoothie

• Some icons are rendered in black, which does not have enough contrast against the background
  

• There is no active indicator on the search bar
  

• When changing the theme to Pitaya Smoothie, the bottom bar disappears
  

Compared to GH dark