I use these few simple styles for Gmail in the Chrome extension Stylish to help make Gmail a little easier to parse visually.

stylish-chrome-ext-gmail-styles.css

/* page header */
.gb_uf.gb_tg {
  background-color: #3D4957;
}

/* read message rows */
.yO {
  background: #EAF2FE;
  color: rgba(78, 104, 139, 0.61);
}

/* compose button */
.T-I-KE {
  background-color: #FF7E5E;
  background-image: none;
  border-color: #E06141;
}

/* direct mention badge */
.ar.as .at[title*="Direct Mention"] {
  background-color: #FF7E5E !important;
  border-color: #FF7E5E !important;
  border-radius: 2px !important;
}
.ar.as .at[title*="Direct Mention"] .au {
  border-color: #FF7E5E !important;
}
.ar.as .at[title*="Direct Mention"] .av {
  color: white !important;
}