css/main.css

line 998 (change .75 from 0 to 1):

.pix-header.white.menu-singleblock.fixed,
.pix-header.white.menu-aroundblock.fixed,
.pix-header.menu-aroundblock.pix-levels.white.fixed .pix-header-menu .container{
    background-color: rgba(255,255,255,.75);
}

line 1137:

.pix-header.fixed .container, .pix-header.fixed .container-fluid {
    height: 50px;
}