Dorothea is from the Greek dōron (gift) + theos (god) — same elements as Theodora in reverse order. Dorothea Brooke is the heroine of George Eliot's Middlemarch (1871-72) — widely considered the greatest English novel.
Dorothea 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 Dorothea reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.