Preload fonts added via custom.css

We're adding some custom fonts via `custom.css`. Unfortunately it would appear that the fonts are not preloaded, and there is no easy option available for us to make them so. What could be done to ensure custom fonts are preloaded?

For clarity, [a link to the sorts of optimizations we are hoping to achieve](https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/webfont-optimization#optimizing_loading_and_rendering).

Start by tracing how custom.css is loaded and how custom font URLs are handled, then compare that behavior with the linked webfont optimization guidance. The work is complete when fonts declared through custom.css can be preloaded without requiring an inconvenient manual option, with the resulting behavior verified in a browser.

Written by the indexing model from the issue text.