One issue could be the serving up of resources over HTTPS. For example, you main site contains very many absolute paths. Maybe you had a reason for this, but it can lead to problems :)
If you change:
href="http://[whatever the uri is].css"
To:
href="//[whatever the css uri is].css"
You will then have 'protocol relative' paths.
Another pointer is using Firefox with the FireBug extension installed. You can find it here
Finally, under WordPress General Settings you can change the WordPress Address (URL) and the Site Address (URL) from HTTP to HTTPS. If you don't have access to your admin you can change this via your wp-config.php, see here for guidance on doing so.
EDIT
The issue is the paths supplied for your resources.
Try right clicking in the browser and selecting View Source
. See the tag of your page. It contains all stylesheet and javascript includes.
At present, all paths are prefixed with http://
which will not work over the HTTPS protocol. My point here is to use protocol relative paths.
For example, in the Inspector (within Firefox) I changed the following line:
<link rel="stylesheet" id="twentyfifteen-style-css" href="http://www.mrxidevelopment.de/wp-content/themes/twentyfifteen/style.css?ver=4.2.4" type="text/css" media="all">
To
<link href="//www.mrxidevelopment.de/wp-content/themes/twentyfifteen/style.css?ver=4.2.4" rel="stylesheet" id="twentyfifteen-style-css" type="text/css" media="all">
The change rendered your page in my browser like this:
Note the href=
attribute was the only change - but it enables successful location and load of the CSS file.
You will need to do the same for any other resources you wish to include (i.e. Images, CSS and JS etc.)
For more information see the SO questions here and here.
Just for clarity
Changing the href
attribute within the Inspector (or other browser tools) is not permanent - but is a great for debugging.
To permanently fix the paths open up your source in an editor Sublime Text is great for this and edit your mark-up.
Let me know if you need any further help mate.