Add CSS variables to status icons so that the background may be changed

Issue: #120 (closed)

Right now all the status icons have a white background, which causes problems with changing the color of the icon, and putting the icon on a background other than white. This MR adds a CSS variable to each of the icons that use a white background so that the background could be modified in CSS later.

Below is a screenshot demonstrating the usage:

Note: to see the effect (on the dev server), you'll need to modify the background to be grey on the tiles that hold the icons, and also define "--svg-status-bg" css variable and give it a value, so that the change is noticable.