You can simply add another CSS definition.
.dhx_view{
height:374px;
}
This will override the previous CSS definition as long as it is loaded after the first in the HTML page. It will also keep the other rules in that style and just override the height.
Also, if you're going to assign a CSS class to an element, put all of the CSS in there. I wouldn't mix inline styles with stylesheets. It's just easier to maintain that way.
Edit:
I originally thought that the element.style was CSS that you had defined [which repeats the inline styles] but I think it may for reference and not code. In that case, you can't just add another CSS class to override the original definition because - as a few people pointed out - the inline styles can't be overwritten by CSS, without using the !important declaration.
So your options are to use CSS without inline styles, or use inline styles with the !important flag.
!importantafter propery value e.g..dhx_view { margin-top: 0 !important; }