Reset Style Sheet Overrides Theme CSS

The way you’ve implemented your reset.css file /my-calendar/css/reset.css causes it to override theme styles.

For example in one of our stock themes we have the body line-height set to 1.5625 but because plugin styles are added after theme styles your reset style sheet overrides that line height and sets it to 1. That change breaks the theme.

Please make your plugin CSS styles more specific so they only apply to the content added by your plugin instead of applying to everything on a site.

https://wordpress.org/plugins/my-calendar/