Sometimes you want some transparency.
You can make the polybar transparent and it is not that difficult.
There are 3 color themes present 01/2019 and only one is active.
Change this line of code from
background = #2F343F
background = #aa2F343F
Line 43 in the image.
The aa stands for the amount of transparency.
Capital or small letters do not matter now.
FF is full opaque or not transparent.
00 is fully transparent or no coloring at all.
These are hexadecimal numbers.
They range from 0, 1, 2,.., 9, A, B, C, D, E, F
The system tray bar with all the icons will never become transparent.