SteveM Hmmm..... I'm having trouble finding something that will work. It turns out the one I'm seeing, clarkwinkelmann/flarum-ext-high-contrast (v0.1.0, released April 1 2018: Initial version), is unfortunately what you might expect based on the release date. It's probably great - if you want all the text to be rainbow Comic Sans.
It's a bummer because the developers are aware this is an issue with the default theme (I found discussions about contrast levels from 4-5 years ago.) They did increase the contrast some but not up to recommended levels for visually impaired folks. They felt it could be addressed by an extension or by modifying the CSS.
I'm looking into other possibilities and if you're aware of another extension, I'm open to it. Otherwise, I'll see what I can do with overriding some of the CSS to improve the situation. In the meantime, I know some of the web browsers have add-ons to increase contrast.