I get it that it just sounds nice, but what's the theory behind it You can make the observation that the blues often uses a minor third over a major chord and then try to make some comparison to this song. But just a stylistic comparison. It's true. And it probably is an appropriate comparison. But it doesn't explain why musicians do it. I think an important theory idea to consider is the difference between tonal scale degrees and modal scale degrees. The tonal degrees are the tonic and the perfect fifth above it and the perfect fifth below it. Those three degrees are present in many different scales and they set the solid foundation for a tonality key or mode. The modal degrees are the third and sixth tones of the scale and to keeps things simple include the second and seventh degrees. These degrees determine the key/scale/mode like major, Phrygian, Freygish, etc. etc. In very broad terms the tonal degrees stay in place, but the modal degrees can change around in various ways. In this song there is a D major chord using F, but there is also an F major chord the root of which is F natural. Also, while the D major chord is played there are melodic tones of F natural so that F and F natural sound simultaneously. The changing F and F natural is the first concern. Generically those tones are the third scale degree above D, the mediant of D, a modal degree. This fits nicely into our general concept that tonal degrees stay fixed but modal degrees are variable. Secondly the F and F sound simultaneously. That may seem strange, because relative to each other the two tones are a very dissonant minor second. When two tones differ chromatically like this, but occur in close proximity or simultaneously is can be called a false relation or cross relation. In various "classical" music styles there are sometimes false relations in the minor more between form of the sixth and seventh degrees. The blues does something similar, but with the third degree. But that's dropping back into stylistic comparisons. Why is this clashing of false relationships acceptable? I think the answer is that while to the tones seem to "contradict" each other they make sense as belonging to individual parts. Parts here means contrapuntal or melodic parts. In classical style if there is a false relationship of the sixth or seventh degrees it's usually because two parts are moving in opposite directions and direction is a factor determining the quality of those degrees. In blues and rock the quality of the third is usually separated by part. The guitar may play the major third in accompanying chords, but the voice sings a minor third in the melody part. If the parts are considered separately, you will normally find nothing unusual in the treatment of tones. Only when the parts are combined does the clash become apparent. You could say the integrity of the individual parts trumps the "vertical" combination of parts. Which is sort of like the classic counterpoint versus harmony view. In this song - if I'm hearing thing correctly, and also watching the singer's left hand on guitar in the video - the guitar part has a smooth chromatic line on the top of the chords F F♮ E F♮, while the refrain of the melody is built around an embellishment of a D minor chord D D D F♮ G A. The integrity of the two separate parts is perfectly clear. One is a slinky chromatic line, the other is a broken triad. Two parts that sound great separately go together and we don't mind the dissonant false relationship. So, the theory behind it... modal scale degree are variable in many styles of music dissonant false relations work when the clashing tones make sense within individual parts dissonance works when handled properly