Translations:New Template System in LS3.x/58/en

The easy way: using Google Font CDN
Bootswatch Survey Theme uses Google Font CDN. Let's have a look to how it works: https://github.com/LimeSurvey/LimeSurvey/blob/70a1d99cd2ebe411597231a8bc746b4ca9e19584/themes/survey/bootswatch/css/variations/flatly.min.css