The marker timeline labels are difficult to read (Low vision accessability)

The marker timeline canvus (click the Timeline button and look down) where the markers are drawn on a horizontal timeline, has difficult to read labels for the markers. My firefox is configured to ignore the colours of a site and substitute my own high contrast inverse colours. When this happens the titles on the left of these markers have some poor contrast and can be difficult to read. I would like them to have white-on-black, and perhaps a larger font.


┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-668)