This is the documentation page for Module:TemplateStyles

Exports a function that takes the name of CSS page (in en.wiktionary it is 'a sanitized' css) and
returns a <templatestyles /> tag.

In Module:xxx, require("Module:TemplateStyles")("Template:xxx/style.css")

e.g. check last line at wikt:en:Module:character info
(See also the mw:TemplateStyles for the TemplateStyles extension.)