Should I write "module/theme" or "module / theme"?

You should remove the spaces. Unless, of course, you are quoting a poem, in which case the slash indicates a line break:

We review a module
theme per user.

Wikipedia has more info:

There are usually no spaces either before or after a slash. Exceptions are in representing the start of a new line when quoting verse, or a new paragraph when quoting prose. The Chicago Manual of Style (at 6.112) also allows spaces when either of the separated items is a compound that itself includes a space: Our New Zealand / Western Australia trip. (Compare use of an en dash used to separate such compounds.) The Canadian Style: A Guide to Writing and Editing prescribes "No space before or after an oblique when used between individual words, letters or symbols; one space before and after the oblique when used between longer groups which contain internal spacing", giving the examples "n/a" and "Language and Society / Langue et société".

In print I would leave no space, but for online usage I bracket the "/" with spaces because it is a non-breaking character and results in huge, clunky amalgamations that take up a whole line, leaving the previous line with but a couple of words. This is the kind of break I mean:

If you wanted to use some long words, you could 
go the 
antidisestablishmentarianism/pneumonoultramicroscopicsilicovolcanoconiosis
route.

The two long words won't break at a line end because of the slash, but will if the slash is surrounded by spaces.