DAKboard Supports widget blocks for adding HTML and Javascript code for third-party content. These can be added to custom screens via the add block modal as shown below.
Widget blocks for adding javascript that is sandboxed, meaning they will be unable to create pop-ups for example. Below are some definitions of what should be possible in this block type:
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe#attr-sandbox
You should be able to create your CSS style tags inside but you may find it easier to add them via the Settings & Defaults page like in the below article:
Applying Custom Styles to Screens