It's far from ideal but I was using this https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiQr5qHhKnZAhUBJcAKHU36BxkQFggpMAA&url=https%3A%2F%2Fres-o-matic.en.uptodown.com%2Fwindows&usg=AOvVaw0zm9gFJ085_culLsoheqwS
It allows you to create a shortcut that changes the resolution of that specific .exe. I used it for bigbox setting the resolution at 1920x1080 on my 2560x1080 monitor. Sure it is pillarboxed, but at least there's no stretching or misplaced text, videos, images etc.
Also, If you can live with stretching and don't want the pillarboxing, then if I remember rightly you can set the picture mode/ratio on your monitor to wide. This will stretch bigbox across your whole screen, but all elements of your theme will be displayed in the correct location as if on a 16:9 monitor.
Like I said, not ideal but It's a workaround until a 21:9 theme is made.