Step1: With a click, select the school of which you want to modify the theme.
Step2: As soon as you select the school, a toolbar will appear at the left corner of your screen. Select 'MANAGE' ------>'DOMAINS'
Step3: Click on connect domains.
Step4: Search the Sub-Domain with which you want to connect your Domain and click on the eye icon against it, reading 'Edit'.
Step5: Enter the Domain Name and click 'SAVE DOMAIN'