Searching for 'olDarkTheme' showed entries within each of the translation files located at "\web\app\assets\locales".

Within each of the 'locale_xx.json' files (replace xx with language), there is an entry for theme names which I have previously missed:

  "settings": {
    [ irrelevant text ]
    "themes": "Tema",
    "darkTheme": "Koyu tema",
    "lightTheme": "Açık tema",
    "olDarkTheme": "Openledger koyu",
    [ irrelevant text ]
  },

So it would appear that you need to provide a translation for your new theme in the following format:

"yourThemeName": "yourTranslatedThemeName"

Google translate should be sufficient..

Give me a shout if this works :)