improve(custom.css): support import current graph assets file

[xyhp915]

Use case in custom.css

@font-face {
  font-family: 'Rubik Distressed';
  font-style: normal;

  /* relative path from current custom.css file */
  src: url("../assets/RubikDistressed-Regular.ttf") format('truetype');
}

h1 {
  font-family: 'Rubik Distressed' !important;
}

Mentioned thread:
https://discord.com/channels/806040386614853642/840147756487540777/986075938112565299

[andelf]

LGTM

It's useful to have my custom font together with the graph direcotry.