This is the CSS I used for the new Gmail look with Mailplane

Mailplane CSS for New Gmail.css

/* Hide zero inbox message */
.TC[colspan="3"] {
    display: none;
}
/* Hide message sidebar */
.Bu:not(:first-child) {
    display: none;
}
/* Hide categories links in sidebar */
div.byl.aJZ.a0L,
.wT .HwgYue .CL {
    display:none;
}
/* Hide chat */
[gh = "c"] {
    display: none;
}
.YI {
    display: none !important;
}
.Mqxrce {
    display: none !important;
}
/* Hide storage */
.aeV > div > span:first-child,
.aeV > div > div:first-child {
    display: none !important;
}
.aeW {
    visibility: hidden !important;
}
/* Hide legal */
.aeU {
    visibility: hidden;
}
/* Hide account activity */
.ae3 {
    display: none;
}