I have experience in customising Alfresco since the start of Alfresco. Alfresco by default comes with only a few themes which can be customised according to your needs. Here is additional choices when selecting a Theme for their Alfresco or a custom site they are using.
The themes available can be further customised for your organisation needs OR complete new theme can be delivered as per the requirements.
The themes are commercially supported for your Alfresco installation when you purchase and also support other alfresco customisation.