When setting a "theme" setting to "light", only the editor window is changed; the REPL (at least in https://repl.it/repls/HandmadeEdibleVariable for a non-logged-in user) is still on a dark background.
Additionally, this is somewhat difficult to correct with custom CSS (via a browser extension), because the pane is styled directly in the HTML, with e.g.:
div style="z-index: 1; width: calc(50% - 5.5px); height: 100%; background-color: rgb(14, 22, 40);"...
For people and/or environments (e.g. with a projection screen) where light text on dark background is hard to read, it would be really nice if this would honor the theme settings, and/or that the CSS had clean class-based styling that could then be overridden easily.