CSS - place block in the middle

CSS - place block in the middle

I have a simple PHP website, that uses PHP only to include another files
in index.php, so you may consider it uses HTML only.
The problem is that when viewing a one page of the site in my laptop, it
seems to display normally, while viewing it from my smartphone or PC makes
it very strange.
Here is the link: strasbourgmeetings.org/rigaCloud/login: you will find
two-color page there with a semi-transparent login form in the middle.
Well the problem is that only my laptop displays it in the middle, but my
PC and other devices are not.
Yes, I know there is a horrible CSS code, but, anyway, I would highly
appreciate your help to place this block in the middle.
P.S.: I thought that top: 50%; left: 50% and margin: -25% 0 0 -25% will
make it centered, but...