Added uxtheme bindings

core/sys/windows/ux_theme.odin

@@ -0,0 +1,12 @@
+// +build windows
+package sys_windows
+
+foreign import uxtheme "system:UxTheme.lib"
+
+MARGINS :: distinct [4]int
+PMARGINS :: ^MARGINS
+
+@(default_calling_convention="stdcall")
+foreign uxtheme {
+    IsThemeActive :: proc() -> BOOL ---
+}