Add Custom css Support

[natyusha]

Hi, I've seen a lot of people suggesting various changes that they would like for your excellent dashboard and think that a lot of them could be easily accomplished by simply allowing users to add their own css to a file in the data folder (similar to how the various json files are already used).

Adding something along the lines of <link href="%PUBLIC_URL%/pathtodatafolder/userstyles.css" rel="stylesheet" /> to the index.html would allow users to easily customize the dashboard and solve issues such as these: #19 #37 #50 (with a few lines of !important css)

[phntxx]

Hi, this sounds like an amazing idea. I'll implement this right away!

Kind regards Bastian Meissner