According to How to change the background color of PlatformIO? - #2 by pfeerick PIO should automatically pick up on the used theme and use platformio-dark.css or platformio-light.css. Have you tried to restart VSCode? If that doesn’t work, you can hack the index.html file as referenced in the link and modify the
link.href = './themes/' + workspace + '-' + theme + '.css';
line to e.g. just
link.href = './themes/' + workspace + '-light.css';