I imagine the Custom.CSS failure is related to the fact that its calling for an image. I would remove the two lines related to the Background and see if it works then. Other than some colors and of course the forcing of the column widths there is not really much in the custom.css
In the config.js I sent you via email - you will see I changed to using the module MMM-Profilepicture pointing back at localhost to host backgrounds there. It is much cleaner easier to use. I also switched from wunderlist to todoist for the todo list…
I have not done much other than change the background for many years now - Wife is asking for a refresh, and we picked up a Narrow hutch someone was throwing away that with a coat of paint some new handles and a new monitor will make for an excellent info board - so once that is refinished i will start anew again but this time the screen will be in portrait.
Good luck and happy learning