Hi there,
I'm Cherrelyn, , and I completely understand how frustrating it is when a Windows update unexpectedly changes your theme settings. After the July 8 update (KB5056580), Microsoft made it so the “Show accent color on Start and taskbar” option is only available in dark mode and which is why it's greyed out unless dark mode is enabled. This isn’t necessarily a bug but a design change many users have found inconvenient. Unfortunately, there's currently no built-in way to enable accent color on Start and taskbar in light mode. As a workaround, you can temporarily switch to dark mode, enable the accent color, and then return to light mode and though it may not fully retain the change.
I hope Microsoft listens to feedback and brings back more flexibility in a future update.
Warm regards,
Cherrelyn