Nothing to do with light/dark mode, the colors of those boxes are fixed — they use the same background color and text color regardless of palette selection. And you’re right, the background color (#667eea, from the variable --color-feature) is way too light for white text.
Chrome’s contrast checking puts the combination of rgb(255, 255, 255 / 95%) over #667eea at a contrast ratio of 3.46:1, which fails to meet even the WCAG AA minimum of 4.5:1. (Black text, surprisingly, would just squeak by at 4.5:1 — but it still wouldn’t look good or be very readable.)
So, that’s something that needs to be addressed pretty urgently, and honestly indicates that the whole site needs a full accessibility review. It’s 2026, accessibility should be a launch feature for any website.
(In the meantime, changing --color-feature to #3048b6 would make it acceptable as a background, with a contrast ratio vs. white foreground text of 7.12:1, meeting WCAG AAA guidelines of 7:1 or better.)