Gallery height limited to 410px

Hi all,

i’m using WordPress with Jetpack and trying to use and modify the default gallery,
but there’s a problem i can’t seem to solve:

I want to make a slideshow with images that are 1024x780px, but it keeps scaling them down to 410px height, with Inspector i found this code:

#gallery-7-1-slideshow .slideshow-slide img {
max-height: 410px;
_height: expression(this.scrollHeight >= 410 ? '410px' : 'auto');
}

But when i try to overwrite it via custom css, nothing is changed.

Any ideas?