Add the backspace button to the list of memory buttons
The backspace button seems not to be added to any set of buttons, reulting in incoherent font color. This commit adds it to the memory buttons, like the clear button, making the font color coherent with the rest of the buttons and background and foreground colors to be set in the proper section.
Currently, the button is not part of the memory set and font color is not coherent with the rest of the system:
The color of the font is coherent with the system and colors can be set from the settings: