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/options.h

@@ -27,6 +27,8 @@
 #define STARTUPDIR_CUSTOM  2
 #define STARTUPDIR_LAST    3
 
+#define THEMEREGEXSTR      "^(.+)-GTK3-(\\d{1,2})?_(\\d{1,2})?\\.css$"
+
 // Default bundled profile name to use for Raw images
 #ifdef WIN32
 #define DEFPROFILE_RAW      "${G}\\Default"