[6.x] Improve tables in Bard fields, particularly dark mode

[jaygeorge]

Description of the Problem

As per issue #14150, it looks like #14013 made the Bard tables worse in dark mode (not that they were great to begin with – think we might have missed properly styling those!).

What this PR Does

• Makes Bard tables much better in dark mode
• Makes Bard tables better in light mode too
• Closes Tables in a Bard field are hard to read in dark mode #14150

Before - Tables in Dark Mode

Before - Tables in Light Mode

After - Tables in Dark Mode

After - Tables in Light Mode

How to Reproduce

1. Create a table inside a bard field. Add some content and multiple rows. Select multiple cells and use the "Toggle Header Cell" button in the formatting bar. In dark mode, you can't tell whether it's been enabled or not.