Variables not initialized in Class Constructor
First, I would do what Edgar Bonet suggested, call the normal constructor:

MB_GFX_SPI mbGFX (Period, numCathodeLeds, numAnodeLeds, CathodeSSPin, AnodeSSPin);


Second, I'm not keen on what you are doing in the constructor for a variable which appears to be at global scope (mbGFX).

See What's the "static initialization order fiasco"?

Basically you cannot assume that constructors in different compilation units are called in any defined sequence, you also cannot assume that the system has been initialized yet (ie. init() called).

Thus, things that address the hardware (eg. clearScreen(0);) may not work properly, or at all.