Micro-optimization in challenge list to reduce GUI overdrawing

[JPRamirez17]

Introduced micro-optimization in the challenge list by setting the cell labels' background color to be the same as its parent component (table view), in order to minimize overdrawing. Now, when the table cells and its labels are rendered, the underlying pixels that are covered by the labels will already have the same color as the table view background, reducing the need for additional drawing operations.

Results of the micro-optimization is depicted with Color Blended Layers utility in debug mode (red means overdrawing):

my Habitica User-ID: aeb2467a-11d2-4a9e-b1eb-c4120142d38e

[saraolson]

thanks for your contribution! i've recorded it in your Habitica profile and granted you a contributor tier 🎊