The latest version of the CSS file jdownloads_fe.css is contained in a zip file which may be downloaded here to your device.  'Unzip' and use the CSS import to update.

See Documentation for details of importing the CSS file.

http://www.jdownloads.net/documentations/item/import-and-export-of-layouts-and-css-files

This Download is a zip file with all five of the jDownloads 'front end' CSS files:

jdownloads_buttons.css, jdownloads_fe.css, jdownloads_fe_rtl.css, jdownloads_modules.css and w3.css