Move (adapted MyCApp) MUI theme styling to newly created shared library

Description

This task is to use the MyCApp MUI theme, either by:

• Copying the content of cam-mui-theme.tsx directly in the project (this is already done)
• Or by putting it in a library and use that (preferred option)

Further details

This issue is a follow up from the discussion in !18 (comment 711296).

The theme exists in multiple places:

• https://gitlab.developers.cam.ac.uk/uis/devops/digital-admissions/admissions-portal/ap-frontend/-/blob/master/webapp/src/data/cam-mui-theme.tsx
• https://gitlab.developers.cam.ac.uk/uis/devops/ucam-observe/ucam-observe-remote-analytics/-/blob/main/src/themes/cam-mui-theme.tsx

And its about to be introduced in another project too:

• https://gitlab.developers.cam.ac.uk/uis/devops/laboratory-allocator/lab-allocator/-/work_items/396

Part of this ticket is to work together with other teams to create awareness and ensure the theme is common enough to work for every team and every project.

Task list

Acceptance criteria

• Theme is common enough to work in all projects
• Theme works for MUI v5 and v6

Links/references