floating main menu right

First there is a setting on header – header-behavior : “Let Logo And Menu Position Adapt To Browser Window”
( av_header_stretch ) –
but there will be always a distance of logo to the left (in your case) and menu-items ( they all have paddings ) to the right.
Depending on the screen width, the content of the header is set on default to a value less than 100%.
Did you realy want to get rid even of that distance? – than you had to set that width to 100%.

.html_header_top #top .av_header_stretch .container {
  width: 100%;
}

and on responsive case ( 768px or 989px)

@media only screen and (max-width: 989px) {
  .responsive #top #wrap_all #header .container {
    width: 100%;
    max-width: 100%;
  }
}