~/Library/Application Support/Google/Chrome/Default/User StyleSheets/Custom.css

Custom.css

/* -- Gmail -- */
/* mailbox selector thing */
.W0F9rd,
.J-N-Jz {
  color: #28701a !important;
}
/* compose button */
.T-I-KE {
  background-image: -webkit-gradient(linear,left top,left bottom,from(#4BaD39),to(#48a136)) !important;
}
.T-I-JW {
  border: 1px solid #28701a !important;
}
/* Mail, Inbox sidebar links */
.z8 .is,
.nZ.TO .n0,
.akh {
  color: #4BaD39 !important;
}
.asT-asx {
  -webkit-filter: hue-rotate(100deg);
}
.ain {
  border-color: #4BaD39 !important;
}
.fy1Lpf {
  border-color: #28701a !important;
}
/* Mailbox focus bar - Make it thicker and more noticeable */
td.PF {
  width: 8px !important;
}
/* Mailbox focus row - add a hover color */
tr.NBcQl {
  background: #e8e8e8 !important;
}

/* -- Calendar -- */
/* -- left sidebar containing logo and "create" button -- */
#mainlogo {
  color: #4BaD39 !important;
}
.qnb-container .goog-imageless-button, .action-btn-wrapper .goog-imageless-button, #df-fdbk {
  background-image: -webkit-gradient(linear,left top,left bottom,from(#4BaD39),to(#48a136)) !important;
  border: 1px solid transparent !important;
}
.qnb-container .goog-imageless-button-hover {
  border: 1px solid #28701a !important;
}

/* -- today column -- */
.wk-today {
  background-color: #4BaD39 !important;
  border: 1px solid #28701a !important;
  color: #fff !important;
}

.st-bg-today,
.tg-today {
  background-color: #e1ffb3 !important;
  border-left: 1px solid #28701a !important;
  border-right: 1px solid #28701a !important;
}
.tg-col-today {
  border-left: 1px solid #28701a !important;
}