Making the codified theme file name mandatory (#3446)

...which mean that TooWaBlue.css can't be loaded until it has been
reworked and renamed to the new mechanism. The default theme will be
loaded instead.

rtgui/preferences.cc

@@ -41,7 +41,7 @@ Preferences::Preferences  (RTWindow *rtwindow)
     , parent (rtwindow)
     , splash (nullptr)
 {
-    regex = Glib::Regex::create("^(.+)-GTK3-(\\d{1,2})?_(\\d{1,2})?\\.css$", Glib::RegexCompileFlags::REGEX_CASELESS);
+    regex = Glib::Regex::create(THEMEREGEXSTR, Glib::RegexCompileFlags::REGEX_CASELESS);
 
     moptions.copyFrom (&options);