Some resources (like logos and images) are available, with sources, in the directory doc or images. All the resources in this repository are provided under the CC BY-SA license. So you can use, modify and redistribute the resources, even commercially.
To access 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 page which holds all the documentation.