Reduce gallery caption size

[JustinyAhin]

Fixes #238

Summary

Reduce size of gallery caption.

Relevant technical choices:

No

Test instructions

This PR can be tested by following these steps:

1. Add a gallery block to a post or page
2. Add a caption to the image
3. Select one of the link options in the block sidebar
4. Set the gallery so that the images are fairly small
5. For galleries that have three column and more the font size of the caption is now defined to --global--font-size-xxs which value is 0.75rem.

Quality assurance

• I have thought about any security implications this code might add.
• I have checked that this code doesn't impact performance (greatly).
• I have tested this code to the best of my abilities
• I have checked that this code does not affect the accessibility negatively

[kjellr]

I'm a little confused why this PR includes two size rules, and introduces a new font size variable. Can you explain the reasoning there? From the issue, it sounds like the caption size should just be changed to --global--font-size-xs, which is an existing variable.

Also — when changing the font size here, be sure to change it in the editor styles as well.

[melchoyce]

From the issue, it sounds like the caption size should just be changed to --global--font-size-xs, which is an existing variable.

Yes, we should go with this instead of introducing anything new.

[JustinyAhin]

@kjellr @melchoyce I introduced the new font size because with --global--font-size-xs which is 1rem, the caption still covers all the image.

@aristath mentionned to me that a size under 16px could introduce accessibility issues. So I reverted and use the --global--font-size-xs.

[kjellr]

Thanks, I think this works fine for now. 👍