Better organization of color, fonts and logo, and support for easy customization

[a-limyr]

There is a need for an update of the way the application handles styling. As of now the colors, sizes, fonts and logos should be more centralized. The code as it is now is too distributed in this.

A benefit in centralizing the styling would also be that we can make a set up that will handle easier change of styling. The idea would be to set it up so that there is a default color scheme, but you should also easily bring your own if you want.

[testower]

We should take advantage of the theming capabilities of Material-UI (https://mui.com/material-ui/customization/theming/)