Bug 1843238: Make List-group-item-text background transparent

Fixes list-group-item-text background from white to transparent.
An example of where this appears is in the "Latest changes I can view"
block. The page description had a white background, which did not look
good as there shouldn't have been a background.

behatnotneeded

Change-Id: Ib5b08d1a49b6c22458fbf8e0dfe846a00920ff22