if you use a joomla mixed page with http and https all css will load through http, cause JURI::base() will not use https.
Here is a small solution to fix it:
This will solve the problem.
Also in lightbox.js is still a issue:
var loadingImage = 'components/com_jdownloads/assets/lightbox/loading.gif';
var closeButton = 'components/com_jdownloads/assets/lightbox/close.gif';
var loadingImage = '/components/com_jdownloads/assets/lightbox/loading.gif';
var closeButton = '/components/com_jdownloads/assets/lightbox/close.gif';