Returns one of the following strings to indicate the default visual
class for _w_i_n_d_o_w's screen: ddddiiiirrrreeeeccccttttccccoooolllloooorrrr, ggggrrrraaaayyyyssssccccaaaalllleeee, ppppsssseeeeuuuuddddooooccccoooolllloooorrrr,
ssssttttaaaattttiiiiccccccccoooolllloooorrrr, ssssttttaaaattttiiiiccccggggrrrraaaayyyy, or ttttrrrruuuueeeeccccoooolllloooorrrr.