Fix 2140: Use $loading-backdrop-* SCSS values

Issue #2140 

This makes the styles for the loading mask's backdrop and message box use the values from SCSS variables (which were previously ignored/unused), and updates the values of those variables to match the effective styles that those elements received before.

There is one visible change here: Previously the backdrop would transition in 0.1s while the loading message would transition in 0.2s. If this gets merged then both will be 0.2s.