Make the new gmail style less... urgh. Doesn't fix the buttons missing a background, or similar material design bullshit. Note: It may make some features inaccessible (the addon sidebar which this hides contains links to "Calendar", "Keep", and "Tasks" by default). This is my own stylesheet, shared in case others find it useful, in whole or in part

fix-new-gmail-style.userstyle.css

@-moz-document domain("mail.google.com") {
/* Get rid of right-hand "addon" sidebar */
.bAw {
    display: none;
}

/* Shrink left sidebar to a sensible size */
.aeN {
    min-width: 202px !important;
}

.wT {
    /* This should match the original difference - makes room for scrollbar */
    max-width: 187px !important;
    min-width: 187px !important;
}

/* Fix font size in email list */
.zA > .a4W, .xY {  /* Most of the text */
    font-size: 80% !important;
}

.yW {  /* Make the from part behave */
    font-size: inherit !important;
}

/* Disable the aggressively bold font */
* {
    font-family: /*Roboto,RobotoDraft,*/Helvetica,Arial,sans-serif !important;
}

/* Make the email count ("1-50 of 162") more visible */
.Dj, .adl {
    color: rgb(32, 33, 36) !important;
}

.Dj .ts, .adl .ts {
    font-weight: bold;
}

/* Make the email list item height slightly more compact */
.zA {
    /* This actually ends up 1px taller than the old design, but close enough */
    padding-top: 3px;
    padding-bottom: 3px;
}

/* Make the left-hand menu stay the fuck down */
.HwgYue {
    display: inherit !important;
}

.n6 {
    display: none;
}
}