Hi. I don't quite understand your request, the button is either pressed or don't, why is there need to change an icon?

When we click the button will switch the auto mode of the spell check in the menu. If we can show up the color of the button with the status of auto mode. Then we don't need check the status on click in he menu and know the auto mode is on of off.
for example:
the auto mode is on, the background color is yellow in the button, the off mode is black.
or
ABC color is white with off, yellow is on

I think the button looks different when pressed but there might be an issue with an icon background. I never released an updated icon for dark theme.
Can you check if it's better with the version of the plugin from here:
https://github.com/Predelnik/DSpellCheck/actions/runs/12978226284/artifacts/2487668902

I check with new version in light and dark mode of the flash with the changed of the button, It doesn't easy to see the changed.
If you want to support with the flash changed two icon file with the background color of the different that will
be useful.
But I suggest it show on the status of the spellcheck with the on and off in auto mode of spell check.