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

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.

I believe the correct usage is word/word unless you're writing a line break in a poem:

Shall I compare thee to a summer's day? / Thou art more lovely and more temperate: / Rough winds do shake the darling buds of May, / And summer's lease hath all too short a date:

Punctuation surrounding a slash is a matter of style. The Chicago Manual of Style, for example, allows for a space on either side of the slash when either of the separated items has a space itself. For your example a space on either side of the slash would be appropriate according to that style convention.

Regarding line breaks, you probably want the front space padding the slash to be a non-breaking space, as starting a new line with a slash would be jolting for a reader.

As a technical writer I need to edit content written by engineers. for some reason they have a tendency to use spaces before and after slashes, and I religiously remove them.

I have started rethinking my inflexibility in this matter as I believe there are times that spaces make the content more user friendly.

A case in point is either/or listings of terms that contain other symbols, for example "PMC_IO51 / XMC_IO_B-9". The spacing makes it clear that the slash is not part of either term.

Normally, no spaces should be used; however, placing a slash with no spaces between two long, polysyllabic words (common in technical writing) makes the sentence harder to scan. This could be one case where ignoring your style guide is justified.

Preferred: We review a module or theme per user.
Optional: We review a module/theme per user.
Wrong: We review a module / theme per user.

When a forward slash is being used to signify options, a general rule is there should be no space on either side of a slash when the alternatives consist of a single word. It is advised to leave a space for better readability on both sides of a slash when the alternatives consist of two or more words or they are complex abbreviations.

Note, however, that the majority of style guides (ACS, AMA, APA, MLA, CMOS—see below) emphasize that slash should be used sparingly and shall never be used to mean legalistic terms “and” or “or”. Slash breaks the reading flow and is poorly positioned on many fonts. Also, in technical writing it is already applied in numerous abbreviations, dates, math expressions, symbols, URLs and code, making its use for options in text more ambiguous.

Therefore, a slash should be replaced with a conjunction to connect the options whenever possible. Relevant quotations from various style guides:

• ACS [1, p. 56]:

  Do not use a slash to mean “and” or “or”.
  incorrect
   Hot/cold extremes will damage the samples.
  correct
   Hot and cold extremes will damage the samples.

• ACS [1, p. 219]:

  Leave no space
  […]

  • on either side of a slash (/)

• AMA [2, p. 472]

  8.4 Forward Slash (Virgule, Solidus). The forward slash is used to represent per, and, or or and to divide material (eg, numerator and denominator in fractions; month, day, and year in dates [only in tables and figures]; lines of poetry). It may also be used in URLs (see 2.0, Manuscript Preparation for Submission and Publication).

  8.4.1 Used to Express Equivalence or Duality. When 2 terms are of equal weight in an expression and and is implied between them to express this equivalence, the forward slash can be retained.

  • Developing skin cancer screening recommendations in the Hispanic/Latino population can be challenging.
  • The diagnosis and initial treatment/diagnostic planning were recorded.
  • There was an excess incidence of prostate cancer, thyroid cancer, and multiple myeloma in the rescue/recovery workers.
  • This is an and/or decision.
  • If the approval process raises concerns among the researchers or the ethics committee/institutional review board members, the author may want to ex- plain the resolution of these issues.

  When the question of duality arises in the he/she construction, change the slash construction when the sex or gender is to be specified; substitute the word or for the forward slash or, preferably, rephrase to be gender neutral.

  • Dr Kate Wolf and Dr Rob Cox agreed to serve on the nomenclature committee. Now I need to know whether he or she [not he/she] will lead the subcommittee on genetic nomenclature.
    -Better: Now I need to know which of them will lead the subcommittee.

  If the sex is unspecified and does not matter, the slash construction is permissible.

• APA [3, pp. 160–161]:

  6.10 Slash

  Use a slash (also called a “virgule,” “solidus;” or “shill”) in the following cases:

  […]

  • to specify either of two possibilities
    and/or (use sparingly)
    Latino/a

  […]

  Do not use a slash in the following cases:

  […]

  • when a phrase would be clearer
    Correct: Each child handed the toy to their parent or guardian.
    Incorrect: Each child handed the toy to their parent/guardian.

• CMOS [4]:

  6.106 Slashes to signify alternatives. A slash most commonly signifies alter­natives. In certain contexts it is a convenient (if somewhat informal) shorthand for or. It is also used for alternative spellings or names. Where one or more of the terms separated by slashes is an open compound, a space before and after the slash can make the text more legible.

  he/she
  his/her
  and/or
  Hercules/Heracles
  Margaret/Meg/Maggie
  World War I / First World War

• MLA [5, p. 77]:

  21h The slash

  […]

  Use the slash sparingly, if at all, to separate options: pass/fail, producer/director. Put no space around the slash. Avoid using expressions such as he/she and his/her and the awkward construction and/or.

• The Craft of Scientific Writing [6, p. 24]:

  A third common source of needless complexity at the word level is the slash (/). The slash is an ugly piece of punctuation that adds unwanted complexity to the language. Examples include he/she, s/he, w/o, and and/or. For the first two examples, you can write she or he or you can use plural pronouns (they, them). In the case of w/o, simply write without. Finally, in the case of and/or, which no doubt a lawyer created, using either or or and suffices most of the time. For those cases in which both are required, you should rely on plain English:

  • Detection calls for an ultrasound or EKG or both.

  You should apply the same stylistic strategy for other concocted slashed terms. In other words, do not write

  • …impact/influence on reservoir quality…
  • …difficult/misinterpreted measurements….

  Instead, either choose one of the terms or use a conjunction to connect the terms:

  • …influence on reservoir quality…
  • …difficult or misinterpreted measurements….

• The Craft of Scientific Writing [6, p. 248]:

  The Slash. As stated earlier in this book, the slash is an ugly piece of punctuation that adds significant complexity to the language. To reduce this complexity, you should avoid word constructions that incorporate slashes. The best solution is to replace the slashed compound word with just a single word—for example, replace demonstrate/show with show. When that possibility does not exist, replace the slash itself with a conjunction such as and or or.

References

1. The ACS Style Guide: Effective Communication of Scientific Information, 3rd ed.; Coghill, A. M., Garson, L. R., Eds.; American Chemical Society; Oxford University Press: Washington, DC; Oxford; New York, 2006.
2. AMA Manual of Style: A Guide for Authors and Editors, 11th ed.; American Medical Association, Ed.; Oxford University Press: New York, 2020.
3. Publication Manual of the American Psychological Association, 7th ed.; American Psychological Association (Washington, District of Columbia), Ed.; Washington, 2019.
4. The Chicago Manual of Style, 17th ed.; University of Chicago Press: Chicago; London, 2017.
5. Hacker, D.; Sommers, N. I. A Pocket Style Manual: 2016 MLA Update; 2016.
6. Alley, M. The Craft of Scientific Writing, 4th ed.; Springer: New York, NY, 2018.