Although custom themes are not supported, you are still able to use custom HTML, JavaScript, and CSS code inside Appit.  

To do this, add a DataLabel control to your view or form and add the HTML inside there.

There are two downsides using this in Appit.

1. Again, it will not be supported.

2. Due to it being tied to a Data Label control, you are limited to 255 characters of text.