Fix minor load/set glitches
Give OS_GBPB a larger buffer to enumerate the themes, and post filter those that are too long for the gadget (rather than bailing out with a buffer overflow error). Correct a discrepancy between the gadget buffer size and #define. Correct the fallback button well colour (when not given explicitly) to not be light blue, due to WimpVisualFlags wanting RGB values rather than palette entries.
See https://www.riscosopen.org/forum/forums/1/topics/16369?page=3#posts-125961 and https://www.riscosopen.org/forum/forums/4/topics/16741 and https://www.riscosopen.org/forum/forums/5/topics/16708#posts-125707