Customize styles at deploy time

[noelalonso]

If i build the image, i can copy my css to /src/theme/custom.scss and customize the style. However, if I use ghcr.io/radiantearth/stac-browser, I can't do it at deploy time. This option would be interesting to be able to apply other styles, icons, etc.

[m-mohr]

Please provide a PR. As I'm not a user of Docker, I won't work on it.