In DNA's double helix configuration, thymine is always paired with the base adenine. (In RNA, thymine may be paired with uracil, but other configurations are possible.)
As seen here, thymine is shown in yellow using Foldit's default "hydro" coloring. It's considered "large yellow", and adenine is "small yellow". (Cytosine is "small purple", and guanine is "large purple".)
The rule for DNA base pairing is that small goes with large, yellow goes with yellow, and purple goes with purple.
See the DNA Gallery for all four DNA bases.