Character data type. If Themes is not turned on or if you are running an older operating system that does not support Themes, SYS(2700) returns a value of 0 when arguments are omitted. If Themes is turned on, SYS(2700) returns a value of 1.

The behavior of calling SYS(2700) without any arguments to return the current setting differs from the _SCREEN system variable in that it always returns the state of Themes on the computer. Therefore, if you use SYS(2700,1) to turn on Themes for Visual FoxPro and disable Themes using the Display control panel in Windows, SYS(2700) returns a value of 0 whereas _SCREEN.Themes still returns True (.T.).

When you change Themes support using the SYS(2700) function, Visual FoxPro might not refresh the user interface automatically. However, when Themes is turned on, certain effects such as those that appear when moving the mouse over controls will work. Therefore, it is recommended that you set Themes support at the beginning of an application before the user interface is rendered.

Note
If you change Themes support while a form is running, you can call the form's Refresh method to redraw themed controls.

Reference