Hi,
What SP is the system on?
If this is happening in all browsers then please reproduce the issue with the below components logging severity increased to ALL, capture a logwatch trace as described in kba 1593107 and paste any theme related exceptions.
com.sap.portal.design.*
com.sap.portal.design.StyleArchive
com.sap.portal.design.StyleEditor
com.sap.portal.design.StyleService
Thanks
Lorraine