Sample CSS for Control Panel (Craft 3)

cp.css

/* Sidebar background color */
#global-sidebar {
    background: #333f4d;
}

/* Header background color */
#main-container #main #header {
    background: #ebedef;
}

/* H1 tags */
h1 {
    color: #29323d;
}

/* Standard button color */
.btn.submit {
    background: #da5a47;
}
/* Hover button color */
.btn.submit:not(.disabled):not(.inactive):hover,
.btn.submit:not(.disabled):not(.inactive).hover {
    background: #bf503f;
}
/* Active button color */
.btn.submit:not(.disabled):not(.inactive):active,
.btn.submit:not(.disabled):not(.inactive).active {
    background: #8c3b2e;
}