This is a refinement for Gmail's 2018 redesign. It brings a whole lot more color into the interface and fixes some whitespace issues I had.

gmail-redesign-refinements.css

/* --- Topbar ---*/
/* Background */
.nH.oy8Mbf.qp {
	background: #D64541;
}

/* Icons */
.gb_ja svg {
	color: rgba(255, 255, 255, 0.6) !important;
	transition: all 0.4s;
}

.gb_3b:hover svg,
.gb_6d:hover svg,
.gb_x:hover svg {
	color: rgba(255, 255, 255, 1) !important;
}

/* Gmail Logo */
a[href="#inbox"][title="Gmail"] {
	position: relative;
	top: -2px;
	width: 50px;
	padding: 4px 0 1px 9px;
	border-radius: 8px;
	background: rgba(255, 255, 255, 0.1);
	transition: all 0.2s;
	overflow: hidden;
}

a[href="#inbox"][title="Gmail"]:hover {
	background: rgba(255, 255, 255, 0.4);	
}

/* Search */
form[role="search"][id="aso_search_form_anchor"] {
	border-radius: 3px;
	background: white !important;
}

.gb_Se .gb_pf.gb_rf {
	position: absolute;
}

.gb_rf.gb_lf {
	background: white !important;
}

/* --- Left slide-out panel --- */
.aj9 {
	border-right: solid 1px #EEE;
}

.aim {
	padding: 4px;
}

.z0 {
	display: flex;
}

.bym .z0 .T-I {
	flex: 1 !important;
	margin-right: 10px !important;
	transition: all 0.2s, box-shadow .08s linear,min-width .15s cubic-bezier(0.4,0.0,0.2,1) !important;
}

.bym .z0 .T-I:before {
	transition: all 0.4s;
	transition-timing-function: cubic-bezier(0.68, -0.55, 0.265, 1.55);
}

.bym .z0 .T-I:hover {
	background: #D64541;
	color: #FEFEFE;
}

.bym .z0 .T-I:hover:before {
	transform: rotate(270deg)
}


/* --- Main view top header --- */
.aeH {
	border-bottom: solid 1px #EEE !important
		;
}

.G-atb {
	height: 60px !important;
}

.G-atb::before {
	-webkit-box-shadow: none !important;
	box-shadow: none !important;
}

.aRz.J-KU {
	border-right: solid 1px #F5F5F5 !important;
}

/* --- Mail preview snippets --- */
.Cp table tr {
	padding: 14px 0 !important;
}

/* --- Mail preview attachments --- */
.brd {
	margin-top: 10px !important;
	margin-bottom: 1px !important;
	margin-left: 0 !important;
}

.brc {
	border-radius: 4px !important;
}

/* --- Mail header --- */
.nH.br6.byY {
	padding-top: 26px !important;
	padding-right: 16px !important;
	padding-bottom: 20px !important;
	background: #FBFBFB;
	border-bottom: solid 1px #EEE;
}

.ha>.hP {
	font-size: 24px !important;
	color: #555;
}

/* --- Mail --- */
.nH.if {
	margin-right: 0;
}

.gH.bAk:last-of-type {
	margin-right: 16px;
}

tr.acZ td:last-child {
	margin-right: 16px;
}

.kQ {
	background: #FBFBFB;
}

.adv .adx {
	background-color: #FBFBFB !important;
}