add a macro for checking if default theme is set #1936

Job	Run time
build	58s
build-lang	28s
build-variants (EXTRA_FEATURES=0, PADEMU=0)	1m 0s
build-debug (iopcore_debug, :main)	1m 12s
build-variants (EXTRA_FEATURES=0, PADEMU=1)	1m 17s
build-debug (ingame_debug, :main)	1m 9s
build-variants (EXTRA_FEATURES=1, PADEMU=0)	1m 5s
build-debug (eesio_debug, :main)	1m 30s
build-variants (EXTRA_FEATURES=1, PADEMU=1)	1m 12s
build-debug (iopcore_ppctty_debug, :main)	1m 7s
build-debug (ingame_ppctty_debug, :main)	1m 18s
build-debug (DTL_T10000=1, :main)	1m 11s
merge-variants	7s
release	0s
merge-debug	5s
	13m 39s

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