NAT-310: GEANT colors

I cannot manage to run the dockerized GUI properly myself because for some reason the environment variables are not being applied despite being correctly passed.

jorge@jorge03:~$ sudo docker exec -it 8 /bin/sh
/app # echo $REACT_APP_BACKEND_URL
"http://192.168.200.202:3000"
/app # echo $BACKEND_URL
"http://127.0.0.1:8080"

So what I did is, I checked the changes in a local copy of the orchestrator-core-gui repository, and then applied them in the Dockerfile by copying the required CSS files that had to be changed from the original. I built the image to make sure there were no errors. I had to change a bit the suggested color for the text for the case of the dark theme, as otherwise it could be barely read. You can see the looks here: https://jira.software.geant.org/browse/NAT-310