The hexagon is made with svg, it’s css is in the theme css file:
[host]/[site]/check_mk/themes/[theme name]/theme-[version].css
Look for css selectors with names like path.hexagon.[state] and change the corresponding fill and/or stroke values.
To have your changes easier survive updates, is to add a new theme to Checkmk and make the css changes in your theme. And just keep an copy of that theme as backup.
In this old post I gave some notes how to make your own theme:
Edit:
There is a little bug though when creating new themes, not sure if that got fixed. Canvas graphs get light theme text values assigned even though your new theme is a dark one. See some of my notes on it in this post: