The SYS(2700) Help say:

Return Values
Numeric data type. SYS(2700) without arguments returns a value of 0 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 1 if Themes is turned on.

But testing the value is not correct in some cases:

Win2K SP4 (Spanish) + VFP8:

?VARTYPE(SYS(2700))   &&<--- Returns "C"

WinXP SP1 (Spanish or English) + VFP8 (with & witout Themes):

?VARTYPE(SYS(2700))    && <--- Returns "N" as expected.

Of course it have an relavite easy way to "solve", but I guess it will be easy to fix at SP1.