Header background image: custom css only works on main page?

Running Catch Base Pro at laihianluja.fi/smyo2017. Very simple site :) I managed to set my custom background image for the header background for the main page, but on the other pages the background is just white?! How can I fix this? Here's my Custom css: #masthead { background-image: url("kartta_header2.png"); background-repeat: no-repeat; } What would be the correct id in the css to set the header background image for the entire site? Thanks!