Welcome to the forum @eriku. 
Firstly you need to install @flowfuse/node-red-dashboard in Node-red. You can use the Manage Palette option in the main editor menu.
Make sure that you get this version, not node-red-dashboard which is out of date.
Yes. A template node is prefilled with some sample code to illustrate the three allowed sections, between <template>, <script> and <style> tags. Get rid of all of it.
You have two choices. Either it can all go in the one template node, with the CSS between <style> ... </style> tags or else (generally better) you can put the CSS into a separate template set to be type CSS. In this case you mustn't have <script> tags!
