Basically the mabola skin (https://github.com/EstudioNexos/mabola), but instead dark
Just download the zip: https://github.com/ZordoPlayz/mabola-dark/archive/master.zip
move it into the skins folder of roundcube
extract the zip
rename the extracted folder to "mabola"
remove the zip
choose "mabola" in the roundcube's settings
you're done, enjoy your new material styled dark theme