Themisto (Θεμιστώ) is from the Greek themis (divine law, right) — "right-doing." The third wife of King Athamas of Boeotia, who in jealousy tried to kill her stepchildren but mistakenly killed her own — one of the most tragic figures of Greek myth.
Featured in Euripides' lost Ino.
Themisto does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Themisto reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.