主題
しゅだい · shudai · JLPT N1 · noun
theme
主 (master, lord) + 題 (question, title): the master's question or the theme you are addressing. 'Subject' = what a topic is about.
主 means main or chief, 題 means title or problem. Together they name the main point a text or talk is about, which is why 'subject', 'theme' and 'motif' all fit.
主 — flame/lamp on top of a stand 王
Originally a pictograph of a lamp with a flame on top of its stand, representing the lord or master of the household who tended the fire. The meaning extended from 'lamp/host' to 'master, owner, main, principal'. The character 炷 (wick) preserves the original 'lamp' sense.