Improve contrast of the CKEditor tables

Users reported:

"When editing tables in xwiki, the borders are quite hard to see. They are very light grey."

" in certain light conditions I couldn't see the borders at all. (eventually I made the borders a pixel bigger and black, then copy-pasted that table everywhere)"

Those tables should probably use the 

@text-muted

color instead of whatever is hardcoded in CKEditor...