Them and theme are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Them (pronounced “thuh-emm”) is a pronoun. It is used to refer to a group, in situations where that group is receiving action or attention.
Theme (pronounced “thuh-ee-mm”) is a noun. It means the central point of a story or other media.
The following story uses both words correctly:
Themis was a unique student in her English Literature class, she made the discussions interesting. When the other students generally agreed on the general themes of the books they read, she often disagreed with them.