Is it possible to migrate a branding and themes from previous Plesk versions?
Currently Plesk has no such functionality. Vote for this feature on Plesk User Voice Plesk User Voice portal.
The top-ranked suggestions are likely to be included in the next versions of Plesk.
Branding logo can be uploaded at Plesk > Tools & Settings > Branding.
Plesk Onyx has an option to switch to Plesk 12.0 color scheme in Tools & Settings > Skins and Color Schemes if Skins and Color Schemes extension is installed.