Crossword Clue:

to mark with letters

Answer	Crossword Clue
THEME	Adding certain letters to phrases to mark the 30th anniversary of the seminal "Elk Cloner" computer virus, e.g.
SEMICOLONIZED	Word that could mean "sparsely filled with settlers" or "added a certain punctuation mark to"
PENPAL	Person to exchange letters with overseas

DOGLEGS	Some links holes ... with a hint to the circled letters
STLOUISMISSOURI	City with a landmark spelled out by the circled letters, reading left to right
TILDE	Mark over some Spanish letters
STL	Letters on Mark McGwire's cap
SWAK	Letters in lieu of a lipstick mark
PEA	Mark Dunn's "Ella Minnow ...: A Novel in Letters"