the starting chord of a sequence is not a reliable guide to key (or mode). the reliable guide is your ear: which chord do you hear as the key chord?
You're reading: Theory help with One Headlight by Wallflowers
in this case, the first chord is actually d anyway, and does sound like the key chord to me. it’s very common for rock songs to start on the key chord, at least in an intro vamp before the main sequence, as here. but it’s still a matter of listening for which chord sounds like home – not just looking at the chords and consulting a theory book.
the sequence of the verse starts on g, but (following the establishment of d in the intro) that sounds like the iv to me.
the f# (or f#7), meanwhile, is the normal v chord in the key of b minor. it’s a handy way of distinguishing the key of b minor from d major, with which it (otherwise) shares its chords and notes.
in this case, it’s optional whether we say that means the verse is actually in b minor, or whether it’s still in d with the f# being a “secondary dominant” (v of the vi chord, bm, written as “v/vi”). just depends whether you hear the bm chord as the home chord. so it’s either: iv-i-v/vi-vi in d major or: vi-iii-v-i in b minor take your pick .
the chorus is more clearly in d major, with its g-d-em-a sequence. (iv-i-ii-v.) the em sounds a little like e major, but the bass goes from e to g, and the hint of g# might just be a harmonic of the bass e. (if it was e major, that would be another secondary dominant, v of a, or “v/v”.)
there would be an argument for g lydian if you really thought g sounded like the “home” chord, the point of stability in the sequence. i don’t hear it as that myself. the a of the chorus sounds like it’s waiting to resolve to d at the end, but just
yields to the g of the verse. sounds like a deceptive cadence to me.
the track fades out, so we don’t get a concluding chord at the end of the song, which might help confirm the tonic. but even if they did finish on g, i wouldn’t say i get much of a lydian vibe from this track. sounds like a typical mix of major (d) and relative minor (bm) to me.