Previously we used background images to display tour icons on the nav list and on the tour notification. This caused two pitfalls: First, if fill colors were different inside svg icons, we would see inconsistent tour icon colors. Second, for one tour icon there would need two svg files.
This patch switches to the mask-image approach, which makes a icon svg as a mask filtering the background color beneath, so we can unify icon colors by the external css and remove extra colored svg files.
MozReview-Commit-ID: ErXexz15y8F
f12e7e519f
