Hi @mau5,
If you want to change CSS, the best place to do this is in WordPress’s custom CSS area. You can find it under Appearance > Customize, then open the Additional CSS panel.
If you want to change text in the plugin, like the form labels and error messages, the best way to do this is with a custom translation. I’d recommend Loco Translate, which gives you a nice interface for creating and managing translations:
https://ww.wp.xz.cn/plugins/loco-translate/
(Even if you’re not technically “translating” from one language to another, you can still modify all the text by creating a custom translation.)
If you want to modify the HTML output of the form, you’ll need to be familiar with WordPress’s hooks system of actions and filters. The entire form is defined in an array which you can hook into and modify. You’ll find more details in the source code here:
https://github.com/NateWr/restaurant-reservations/blob/master/includes/Settings.class.php#L803-L910
