Styles adjustments for Gallery captions

[wrenaria]

The captions on WP gallery images are very hard and sometimes impossible to read in their current state. I don't think they need huge adjustments to fix this, here is what I am thinking:

• solid background instead of gradient,
• 50% opacity
• text size 0.875rem to match the size of regular captions.
• appx 8px padding around the text

Current	Approximate Goals

Example from https://library.ucdavis.edu/exhibit/ground-beneath-our-feet-nikola-p-prokopovich-papers-on-land-subsidence/

Still hoping for lightbox feature at some point too to improve image viewing (but the caption styling change above is more pressing in the short term for site readability).

[spelkey-ucd]

@wrenaria caption style changes on sandbox:

https://sandbox.library.ucdavis.edu/exhibit/ground-beneath-our-feet-nikola-p-prokopovich-papers-on-land-subsidence/

[wrenaria]

Looks good to me!

[spelkey-ucd]

deployed to website_tag: v3.1.0