Integrating Autolab Dark Theme

[fanpu]

There is an Autolab dark theme PR which allows each individual user to set whether they want the default Autolab theme, or the dark theme: #1139

This is a very promising PR but there are some issues with it which needs to be resolved before it can be merged.

1. Certain aspects of the light theme (the handin-panel, the font colors and type, the handin table on submission page) were overwritten for convenience in the dark theme implementation. This should not be the case. There should not be UI changes for the current light theme.
2. The naming conventions of the CSS variables, as well as how the CSS variables are managed can be improved. Hex codes representing the color are being used as the name of the color, but this is very uninformative and decreases readability.