Having a BAD HAIR DAY, so I'm Off to the Salon: This theme threw me for a moment until I wrote down each of the answers and stared at them for a few moments. The last word of each theme answer describes what can be done to one's hair. We had a BAD HAIR DAY last Friday, so today, after getting Waves, Highlights, Treatment, Extensions and Streaks, we'll look Great!
17-Across. Patterns recorded by an EEG: BRAIN WAVES.
23-Across. Items on a TV headline ticker: NEWS HIGHLIGHTS.
35-Across. Passive-aggressive refusal to speak: SILENT TREATMENT.
49-Across. .doc and .pdf, for example: FILE EXTENSIONS.
56-Across. Uninterrupted periods of success: HOT STREAKS.