gmail-dark.css

/* Extra styles for Gmail applied with Stylus
   in combination with Gmail's dark theme
   and the simpl.fyi Gmail v2 Firefox extension */

/* System fonts for Gmail */
.yW,
.hx .gD,
.xY,
.zA > .a4W,
.zA > .xW,
.yi, .y6,
.a3s, .g6 {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;
    font-size: 1rem;
    line-height: 1.6;
}


/* Override Simplify */
html.simplify:not(.htmlEmail) div[role="listitem"] .a3s {
    font-size: 1rem !important;
}

gmail-light.css

/* Extra styles for Gmail applied with Stylus
   in combination with Gmail's dark theme
   and the simpl.fyi Firefox extension */

/* Max width on email */
table.Bs.nH.iY.bAt {
    max-width: 50em;
    margin: auto;
}

/* Nicer, larger font for email list */
.yW,
.hx .gD,
.xY,
.zA > .a4W,
.zA > .xW,
.yi, .y6 {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;
    font-size: 1rem;
    line-height: 1.6;
}

/* Less prominent tabs bar */
.aKh {
    background-color: transparent;
}