Some resources (like logos, images) are available, with sources, into directory doc or images. All resources on this repository are provided under licence CC-By-SA. So you can use, modify and redistribute them even for a commercial usage..
To go onto the Github dedicated to Dolibarr resources: https://github.com/eldy/sellyoursaas/tree/master/img
Few screenshots are available on this page
If you are looking for documentation, take a look at this dedicated page