About Codecademy: Has it changed there a lot recently? I had problems getting them to understand that I was not interested in the position a few years ago.
Well, they did get acquired roughly year and a half or so ago (i think it was late 2021), so things might have changed from a cultural perspective within Codecademy or the parent company.
One last question if you don't mind - is a light mode with good foreground/background contrast enough for you to see well, even if there are elements with bright colors (e.g. here the orange top bar)? Or is it easier to clearly see everything with fewer colors (and thus higher contrast)?
On the topic of the theme and colors, I think the trick for keeping elements distinct despite low contrast, is to have fewer "levels" of elements. Modern "flat" designs keep very low contrast between the actual page background and any organization elements (containers, panels etc) and only give high contrast to the "main" elements, such as the primary text content and whatever element is currently selected. For example, on a page with a "download" button it's a good idea to give that one button a high contrast scheme, instead of all buttons on the page. Contrast can be done between light and dark, between gray and color, or between color and a different color.
Another thought I had about light/dark theme, is that dark theme is often modeled after a colorful console terminal, where technicolor colors are used to draw attention to different types of messages, or as blinking LEDs on a device front panel, so that gives extremely high contrast between the (basically invisible) bg and the object. Light mode, otoh, is often modeled after a book or a newspaper, or some other physical medium, where there is one basic color for text and the background does the heavy lifting - bright for bulk text, colored for special notices or announements, pale for uninteresting stuff, etc.