Well, this is something in my todo list to migrate the html output of the button to an overrideable layout.
In the same time, i have plans to develop a "framework" classes switch in the future (maybe when i will work on new theme pack system...).
For now, it would need to edit core file admin/utilities/registration/registration.php (which is not recommended).
But, what about using the "custom CSS" option of iCagenda global options (in general settings tab) to set you own css for those classes, and then to have a bootstrap look-like the BS version used on your site ?)
Hope this helps ?
Latest version : iCagenda 3.8.21
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