QStringLiteral / QLatin1String (I still have trouble deciding which to use when); also on the following lines

items in m_theme are parented to this, so this line shouldn't be needed

popup() or show() instead of exec() as to not block?

Showing the exit code reminds me a lot of windows 98, can we show a meaningful error message instead?

whitespace, space after Q_FOREACH (same on a couple of other lines)

Can be simplified by adding QDir::NoDotAndDotDot to the entryList flags

const

const

remove

perhaps on one line? (Not really critical, if you prefer it like this, ignore this comment)

const

merge this and the following line, make it const

exit code should be user-readable (see also above)

I understand that enum class makes this type-safe (i.e. will forbid casting to/from int)

Why's that?

units.smallSpacing?

use units.smallSpacing

i18n()

i18n()

i18n()

we have a couple of those, would be good to at least define this interval somewhere centrally, so we get less hardcoded values all over the code

Encoding key is deprecated, defaults to UTF-8 anyway

should not be empty (it's used for search in systemsettings)

can be removed