An svg dynamic solution for any screen is the following:
<svg xmlns="http://www.w3.org/2000/svg" width="100%" height="100%" stroke-width="1" stroke="#000">
<line x1="0" y1="0" x2="100%" y2="100%"/>
<line x1="100%" y1="0" x2="0" y2="100%"/>
</svg>
And if you want to keep it in background use the position: absolute
with top and left 0.