adding darkest blue colour variable

opentech/static_src/src/sass/abstracts/_variables.scss

@@ -2,6 +2,7 @@ $color--white: #fff;
 $color--black: #141414;
 $color--light-blue: #43bbf1;
 $color--dark-blue: #25aae1;
+$color--darkest-blue: #16729e;
 $color--dark-grey: #404041;
 $color--purple: #8b1fbc;