When someone talks about a dye stealer in a pregnancy test, they are mentioning to a test line that aspects dimmer than the switch line. The test line replies with hCG in urine. The more hCG there is, the dimmer it gets.
This classically arises a few weeks after start, when hCG levels are high. The darker line does not mean whatever bad. In most cases, it is a hopeful sign that the prenatal period hormone is sturdy.