hgk: support the old way of getting the current Ttk theme (issue3808)

It seems like the API has changed somewhere around 8.5.7, so the preferred way
of getting the current theme is now [ttk::style theme use], while the
deprecated (but still working) is $::ttk::currentTheme.

contrib/hgk

@@ -109,7 +109,12 b' bind all <MouseWheel> [list ::tk::MouseW'
 # end of win32 section
 } else {
 
-if {[ttk::style theme use] eq "default"} {
+if {[catch {
+    set theme [ttk::style theme use]
+}]} {
+    set theme $::ttk::currentTheme
+}
+if {$theme eq "default"} {
     ttk::style theme use clam
 }