Support for theming

### Description

Supporting theming would allow service owners to create forms that align with their own visual profile. This means that all colors needs to be defined by a token.

### In scope

_No response_

### Out of scope

_No response_

### Additional Information

Designsystemet already has a theme builder, which can be found [here](https://theme.designsystemet.no/themebuilder?appearance=light&main=primary%3A%230062BA+accent%3A%231E98F5&neutral=%231E2B3C&support=extra1%3A%23F45F63+extra2%3A%23E5AA20&border-radius=4)
More information about theming in Designsystemet can be found [here](https://theme.designsystemet.no/)

### Tasks

_No response_

Start by reviewing the linked Designsystemet theme builder and its theming information, then inspect the frontend project to determine the affected form components and token boundaries. Done should mean service owners can apply a visual theme with all colors defined by tokens, but the issue does not identify files, tests, or a more specific scope.

Written by the indexing model from the issue text.