Resolve "Add colour variables css file"

What does this mr do?

This merge request adds a colour variables scss file such that the colour scheme of the whole application is easily changeable. It changes all the main css files to scss files and they will be compiled into css files with the gradle tasks build, sassCompile or bootRun. Also rewrote global.css to take advantage of sass.

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made.
• Tests were created to test the feature.
• I have updated the documentation accordingly.
• I adhere to the style guide.

Closes #296 (closed)