Login and register on top of eachother css not working

Hi. First of all thank you for this extension.
Its working perfectly except for some problems I have on the checkout page.
Since I have modules on both sides the login en register forms overlap as in attached image-1
I have tried to put move the registration box below the login box as explained in this documentation .

I have tried it in the interface of Hikashop/System/Configuration/Display but nothing happens.
I have also tried it in my own template custom.css but that doesn't work neither.
Everything else I change in my template custom.css is working fine.
I'm using Chrome and Firefox and it doesn't work in both browsers.
I cleared Joomla cache (was empty anyway) and my browsers cache many times.
I even used !important in CSS but nothing works like it should.

When I disable the modules on both sides of the page its working fine next to each other as in image-2 but I want to keep the modules on both sides of the checkout page so its very important for me to have the registration box below the login box.

I also want to get rid of the shipping address at he end of the form but I can't seem to manage that either.
I've been reading for days now on this forum and I read all about a checkbox for 'shipping address is the same as billing address' but these are all old threads so I guess that's not an option anymore?

I hope you are able and willing to help me because the problems on my checkout page are the only things that keeps me from filling my store and putting it online.

Thanks in advance.
.

Thanks Philip.
Meanwhile I got it working on a fresh installation of Joomla and Hikashop.
I placed it in my template custom.css

Maybe if someone else has the same problem, in my case it was:

.hkc-lg-4 {
    position: relative; float: none;
    min-height: 1px;
    padding-left: 5px;
    padding-right: 5px;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

Hello,

Thank you for your feedback !

Best regards,
Mohamed Thelji.