This repository shows an example on how to use customised Milligram CSS library with JS frontend app (example is based on PreactX but the principle applies to any other JS framework).
Application skeleton has been generated with preact-cli v3 and later only lightly modified to remove extra stuff like tests.
Free Google fonts may be downloaded for self-hosting using helper application that allows selection of weights and scripts included in package. This is not required and fonts may be as well loaded with external CSS. This example uses Nunito font by Vernon Adams in latin + latin extended scripts. The reason I included self-hosting fonts in the example is that I could not find any NPM package that provides latin extended script so to make Central European text to look good I'd have to either self-host selected font or load it from external source.