Choosing a different category in the New Ticket page is supposed to reload the page. Each category can have different custom fields. The only way for Joomla to reliably load custom fields is to reload the page. Otherwise, some third party custom fields do not work properly.
The category being unselected and the submit button not working are symptoms of the same problem. You did not follow the Quick Setup instructions, most importantly you have not created menu items. Because of the completely insane way Joomla's routing works when you have SEF URLs enabled you cannot have the ticket system work without creating a menu structure for it. I recommend creating a hidden menu with a structure like this:
- Tickets (Type: Akeeba Ticket System, Categories; Access: Registered)
- Category 1 (Type: Akeeba Ticket System, Category; Category: category 1; Access: match the category)
- Category 2 (Type: Akeeba Ticket System, Category; Category: category 2; Access: match the category)
- …
- Latest Open Tickets (Type: Akeeba Ticket System, Latest Open Tickets; Access: whatever appropriate for support staff)
- Assigned Tickets (Type: Akeeba Ticket System, Assigned Tickets; Access: whatever appropriate for support staff)
- My Tickets (Type: Akeeba Ticket System, My Tickets; Access: whatever appropriate for users)
- New Ticket (Type: Akeeba Ticket System, New Ticket; Access: whatever appropriate for users)
This is only needed for URL routing. It does need to be a menu which is published in a module (we don't need it for navigation). In the Manu that's visible to the user just create Alias menu items to the menu items of the hidden menu.
Regarding dark mode, the plugin was only for Cassiopeia which you do not use. We provide the SCSS source files for our CSS in media/com_ats/css. You can of course edit them to match your template and recompile them to CSS using Sass.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!