Create delta note for gt theme
See also
Other gt helpers:
authors_footnote(),
bold_rowgroup(),
caveats_footnote(),
change_footnote(),
dedup_footnote(),
extract_num_colnames(),
legend,
legend_q1,
legend_q2,
legend_q3,
legend_snapshot,
past_fy(),
present_fy(),
present_qtr(),
vlc_footnote()