Hello,

This is possible with custom css (option in global options > General settings) by changing elements of your choice depending on css classes.

By default, the button is using standard bootstrap classes, so the needed css will depend too on your site template, and how it defines its own styles for buttons.

Note too that registration button will be refactoried for next iCagenda 3.8, so custom css today may not work exactly the same later.

Best for giving you help is an online link, to be able to check rendering by your site template.

Best regards,
Cyril