Tweak github theme

dist/github.rstheme

 
 .ace_print-margin {
   width: 2px;
-  background: rgba(3, 102, 214, 0);
+  background: #eff3f6;
 }
 
 .ace_editor {
 
 .ace_marker-layer .ace_bracket {
   margin: -1px 0 0 -1px;
-  border: 1px solid #eff3f6;
+  border: 1px solid #ffeef0;
 }
 
 .ace_marker-layer .ace_active-line {
 .ace_bracket {
   margin: 0 !important;
   border: 0 !important;
-  background-color: #eff3f6;
+  background-color: #ffeef0;
 }
 
 .ace_console_error {
 }
 
 .rstudio-themes-flat .themedPopupPanel {
-  border: solid 1px #eff3f6;
+  border: solid 1px #ffeef0;
 }
 
 .rstudio-themes-flat .themedPopupPanel ~ div.popupContent div:nth-child(1) {
-  border: solid 1px #eff3f6;
+  border: solid 1px #ffeef0;
 }
 
 .rstudio-themes-flat .popupContent {
 
 .rstudio-themes-flat .popupContent:not(.popupMiddleCenterInner) div div:nth-child(2) {
   background: #FFFFFF !important;
-  border: solid 1px #eff3f6;
+  border: solid 1px #ffeef0;
 }
 
 .rstudio-themes-flat .gwt-PopupPanel .popupContent {

src/github.scss

 $ui-selection            : transparentize($blue, 0.85);
 $ui-cursor               : $base0C;
 $ui-cursor-normal-mode   : $ui-cursor;
-$ui-margin-line          : transparentize($ui-selection, 0.2);
+$ui-margin-line          : $base02;
 $ui-gutter-color         : $base03;
 $ui-gutter-background    : $ui-background;
 
 $ui-line-active-gutter    : $light-blue;
 $ui-line-active-selection : transparentize($blue, 0.85);
 $ui-line-find             : $light-green;
-$ui-bracket               : $base02;
+$ui-bracket               : $light-red;
 $ui-invisible             : $base03;
 $ui-indent-guide          : $base02;
 $ui-debug-background      : $light-red;