Headline menu overlaps with the text

Hi,

I am experiencing issues with my headline menu. All texts I insert into different pages are not displayed from the beginning but only from, say, third row. The top of the respective text is invisible – disappearing under the headline menu.
The solution might be to simply add empty rows before the text yet it does not really works as the sizes of overlaps are quite random and also differ depending on the used browser.

Address of the website:

Thank you for the advice!
Antonín