sapgui/theme is a SAP Parameter attribute which is used to control Set the SAP GUI theme to be used with this system information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
The parameter specifies which SAP GUI design the system uses to display
screens.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.