If the reason – incorrect SVG, you need to open your SVG icons file with some editor like Sublime Text or default system text editor and add ()your own) width=”31″ height=”40″ attributes to your <svg> tag like it is shown: http://prntscr.com/m4n0qw
Then you will need to re-upload your icons in WordPress Dashboard.
Sorry but I don’t understand the SVG thing. ¿Where do I have to paste that code?. On the other hand I tried to replace an icon image in the header and it looks pixelated. How can I fix that?