Add _defaults partial to make it easier to add new theme variables

src/rstudio/_defaults.scss

+// primary UI colors
+// $ui-background           : lighten($nord6, 10%);
+// $ui-foreground           : $nord3;
+$ui-code-chunk-background: darken($ui-background, 5%) !default;
+$ui-selection            : transparentize($ui-code-chunk-background, 0.6) !default;
+$ui-cursor               : #FF0000 !default;
+$ui-cursor-normal-mode   : $ui-cursor !default;
+$ui-margin-line          : lighten($ui-selection, 5%) !default;
+$ui-gutter-color         : transparentize($ui-foreground, 0.75) !default;
+$ui-gutter-background    : $ui-background !default;
+
+// less common UI colors
+$ui-line-active           : transparentize($ui-code-chunk-background, 0.6) !default;
+$ui-line-active-gutter    : $ui-background !default;
+$ui-line-active-selection : opacify($ui-selection, 1) !default;
+$ui-line-find             : $ui-selection !default;
+$ui-bracket               : transparentize($ui-cursor, 0.6) !default;
+$ui-invisible             : transparentize($ui-foreground, 0.7) !default;
+$ui-indent-guide          : $ui-invisible !default;
+$ui-debug-background      : lighten($ui-background, 10%) !default;
+$ui-fold-arrows-color     : $ui-cursor !default;
+$ui-fold-arrows-background: transparentize($ui-fold-arrows-color, 0.5) !default;
+
+// rstudio
+$ui-rstudio-background          : darken($ui-background, 2%) !default;
+$ui-rstudio-color               : $ui-foreground !default;
+$ui-rstudio-tabs-inactive       : lighten($ui-rstudio-background, 2%) !default;
+$ui-rstudio-tabs-inactive-color : transparentize($ui-foreground, 0.4) !default;
+$ui-rstudio-tabs-selected       : $ui-code-chunk-background !default;
+$ui-rstudio-tabs-selected-color : $ui-foreground !default;
+$ui-rstudio-toolbar             : $ui-rstudio-tabs-selected !default;
+$ui-rstudio-toolbar-color       : $ui-foreground !default;
+$ui-rstudio-search              : $ui-rstudio-toolbar !default;
+$ui-completions-background      : $ui-background !default;
+$ui-completions-color           : $ui-foreground !default;
+$ui-completions-border          : $ui-bracket !default;
+$ui-rstudio-scrollbar-background: $ui-background !default;
+$ui-rstudio-scrollbar-handle    : $ui-code-chunk-background !default;
+
+// R language colors
+// These need to be set...
+//$rlang-string
+//$rlang-function
+//$rlang-value
+//$rlang-variable
+//$rlang-message
+$rlang-comment  : transparentize($ui-foreground, 0.5) !default;
+$rlang-namespace-font-style: italic !default;
+$rlang-reserved : $rlang-string !default;
+$rlang-operator : $rlang-value !default;
+$rlang-bracket  : $rlang-function !default;
+$rlang-namespace: $rlang-variable !default;
+$other-tag: $rlang-string !default;
+
+// R Markdown
+$rmd-heading-weight: 600 !default;
+$rmd-heading-color: $rlang-string !default;
+$rmd-chunk-header: $rlang-function !default;
+$rmd-href: $rlang-reserved !default;

src/rstudio/_rstudio-dark.scss

+@import "_defaults";
+
 .ace_gutter {
 	background: $ui-gutter-background;
 	color: $ui-gutter-color;
 .ace_meta.ace_tag {
 	color: $rlang-operator;
 }
-.ace_markup.ace_heading {
-	font-weight: 600;
-	color: $rlang-string;
+.ace_markup.ace_heading, 
+.ace_markup.ace_heading ~ .ace_heading {
+	font-weight: $rmd-heading-weight;
+	color: $rmd-heading-color;
+}
+.ace_support.ace_function {
+  &.ace_codebegin, &.ace_codeend {
+    color: $rmd-chunk-header;
+  }
+}
+.ace_markup.ace_href {
+  color: $rmd-href;
 }
 .nocolor.ace_editor .ace_line span {
 	color: $rlang-reserved !important;

src/rstudio/_rstudio-light.scss

+@import "_defaults";
+
 .ace_gutter {
 	background: $ui-gutter-background;
 	color: $ui-gutter-color;
 .ace_meta.ace_tag {
 	color: $rlang-operator;
 }
-.ace_markup.ace_heading {
-	font-weight: 600;
-	color: $rlang-string;
+.ace_markup.ace_heading, 
+.ace_markup.ace_heading ~ .ace_heading {
+	font-weight: $rmd-heading-weight;
+	color: $rmd-heading-color;
+}
+.ace_support.ace_function {
+  &.ace_codebegin, &.ace_codeend {
+    color: $rmd-chunk-header;
+  }
+}
+.ace_markup.ace_href {
+  color: $rmd-href;
 }
 .nocolor.ace_editor .ace_line span {
 	color: $rlang-reserved !important;