Дотемна is an adverb formed from "до" (until) and the root "темн-" (dark), so it literally means "until it gets dark".
It’s a poetic, somewhat old-fashioned word you rarely encounter in everyday speech. In the line «Пропаду в них дотемна» JONY says he’ll lose himself in her eyes until nightfall, creating a vivid, romantic image that makes this uncommon word unforgettable.
This song is a super sweet and direct declaration of love. The English chorus, "My baby I love your voice", tells you the main idea right away. But the Russian verses add so much more feeling! JONY describes his partner as a safe haven from the chaos of the world. He sings, "Буря, буря за окном / Нe войдёт в наш дом" (A storm, a storm outside the window / It won't enter our home), creating this cozy, protected space for just the two of them. Her voice is the key to this safety; he believes "Твой самый нежный голос меня спасёт" (Your most tender voice will save me).
He also uses some amazing imagery to describe how completely lost he is in her. He sees a fire in her eyes and compares them to a fantasy world, singing, "Целый мир как Нарния / Пропаду в них дотемна" (A whole world like Narnia / I'll get lost in them until dark). It's a song about finding someone who feels like both an exciting adventure and a safe place to land, all at once.