* {
    box-sizing: border-box;
}

#header { 
    width: 95%; margin: auto;
    padding: 10px;
    
}
#logo {
    font-weight: bold;
    float: left;
}
#top_menu {
    float: right;
}
.main_block { 
    width:95%; margin:auto; 
} 
.menu_block { 
    width:300px; float:left;  
    padding: 10px;
}
.main_content { 
    margin-left:300px; 
    padding: 10px;
}