2021 Gallery Caption – figcaption

I’m having trouble fixing one teeny, very specific aspect of 2021

The font for captions to images in gallery is much too small, but not adjustable.

I appreciate that fixes are best tackled via child themes, but as I am not very familiar with CSS, I wanted to make sure that I am fixing the right bit of CSS, before introducing too many ways it might not work at once!
I have downloaded the extra config update but it doesn’t offer changing font size in captions in gallery blocks

The appropriate bit of code appears to be (line 2198)

.wp-block-gallery .blocks-gallery-image figcaption,
.wp-block-gallery .blocks-gallery-item figcaption {
	margin: 0;
	color: var(--global--color-white);
	font-size: var(--global--font-size-md);
}

(NB original was var(–global–font-size-xs)

where global font sizes are defined from line 107.

/* Font Size */
	--global--font-size-base: 1.25rem;
	--global--font-size-xs: 1rem;
	--global--font-size-sm: 1.125rem;
	--global--font-size-md: 1.25rem;
	--global--font-size-lg: 1.5rem;
	--global--font-size-xl: 2.25rem;
	--global--font-size-xxl: 4rem;
	--global--font-size-xxxl: 5rem;
	--global--font-size-page-title: var(--global--font-size-xxl);
	--global--letter-spacing: normal;

This is from style.css from the 2021 theme in the subdomain where I am developing this site
changing the font size attribute from xs to md has no effect

Any pointers to what I am missing gratefully received. TIA

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