Wikitable borders use static gray color from our styleguide.
When wikitables cells use background colors, our grey borders don't work well with colors editors have chosen.
E.g.
Solution
Let's use rgba with alpha so the border colors can be the darker shades of whatever colors editors have decided to use in each table.
if we use rgba(0,0,0,0.2) for border here's how the same table looks like
Note: this has no impact on tables without colors
Developer notes
Use variables consistent with the style guide @ https://phabricator.wikimedia.org/source/wikimedia-ui-base/browse/master/wikimedia-ui-base.css
If a variable doesn't exist, make sure the name is consistent and ideally sync with @Volker_E to get it accessible from core.
Be sure to look at the conversation in comments beginning: https://phabricator.wikimedia.org/T168029#3355785