Sets the upper 4-bits of the color attribute which is used for the background color of displayed text.

NOTE: Normally, only the first 8 colors (the highest bit is clear) are available for background usage. The upper 8 colors (the highest bit is set) will cause the text to blink unless the BIOS Blink/Intensity option is set to Intensity.

Examples:

        TextBackground al     ; set background to AL
        TextBackground clRed  ; set background to Red
        TextBackground [LAST] ; set background to value stored in [LAST]

See also: TextColor and TextAttr