Getting to customize an appearance a little I’ve faced with common trouble that the text under images doesn’t center, it always goes to the left side. Additionally, I would like to customize captions text to make it smaller, lighter, and paler.
Brief googling led to www.helpmegeek.com.
As result, I’ve added the following css
code in the Appearance -> Customize -> Additional CSS section:
.wp-block-image figcaption { margin-top: 0px; text-align: center; color: #347B98; font-size: 90%; }
The result likes much better:

Damn, I’m good: the caption is smaller, closer to the image, contrast relative main text, and last but not least – centered.
Not to get up twice added a shadow to the pictures: with pale background color it looks more harmonic from my side, using css
box-shadow
property applied to .wp-block-image img
block.
And as the cherry on the pie: found out the site title text is surrounded with white shadow. OK, I’ve thought, I have instructions: right-click, inspect, add h1#site-title a, #header h1#site-title a:hover
blocks as in original skeleton-theme styles and… works fine, can play a little with effects, maybe remove them all further…
UPD: a:active
doesn’t work though, maybe think about it later.