The Ramblers customisations to Hydrogen has a file called templates/g5_hydrogen/custom/scss/ramblers/icons.scss that refers to ../images/icons/icons-ext.png.
This folder (which resolves to templates/g5_hydrogen/custom/images/icons/icons-ext.png) isn't present in the template.  Only pngs of various devices are there.
The manifests itself as large blank areas (presumably where an icon would be) to the right of hyperlinks to files (eg the pdf files linked to in the text of  this page ).

Does anyone have a copy of the required file?  Or a suggestion about how to fix it?