add theme screenshot
add theme screenshot

HybridAU/pelican-svbhack,Vincent-C/pelican-svbhack,BoldData/pelican-svbhack,eagleoneraptor/blog-theme,manulicious/pelican-hass,MatthewVance/pelican-svbhack,Hernrup/pelican-svbhack,prashanthellina/pelican-svbhack,prashanthellina/pelican-svbhack,gatita/pelican-svbhack-minimal,dnohales/blog-theme,RUDI11/pelican-svbhack,chenstopher/pelican-smpl,laughk/pelican-hss,gatita/pelican-svbhack,gfidente/pelican-svbhack,octogene/pelican-svbhack