The UxTheme.dll is a part of WinXP. The IsThemeActive function returns 0 if themes are not for the current application and non-zero otherwise. See http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/userex/functions/isthemeactive.asp
I guess you're looking for GetCurrentThemeName Function http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/userex/functions/getcurrentthemename.asp

DECLARE Long GetCurrentThemeName IN UxTheme.dll ;
    String @ pszThemeFileName, ;
    Long dwMaxNameChars, ;
    String @ pszColorBuff, ;
    Long cchMaxColorChars, ;
    String @ pszSizeBuff, ;
    Long cchMaxSizeChars

lcThemeFileName = SPACE(512)
IF GetCurrentThemeName( @lcThemeFileName, LEN(lcThemeFileName), "", 256, "", 256) = 0 
    ? LEFT(lcThemeFileName, AT(CHR(0), lcThemeFileName)-1)
ENDIF

If it's not the one, check MSDN at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/userex/refentry.asp


>i found this dll in http://www.davphantom.net/ i can recognize that theme is active or not but i want to know which theme is active?
>because i don't know Spanish i ask here
>
>Declare Integer IsThemeActive in "UxTheme.dll" () As Long
>nRet = IsThemeActive()
>
>? "Themes: " + Iif(nRet = 0, "No activos","Activos")