a good color for open/close bracket indicator is the cursor color

nord-snow-storm.scss

 $ui-line-active-gutter    : $ui-background;
 $ui-line-active-selection : opacify($ui-selection, 1);
 $ui-line-find             : $ui-selection;
-$ui-bracket               : darken($ui-background, 20%);
+$ui-bracket               : transparentize($ui-cursor, 0.6);
 $ui-invisible             : $nord4;
 $ui-indent-guide          : $nord4;
 $ui-debug-background      : lighten($ui-background, 10%);

rsthemes/nord-snow-storm.rstheme

 
 .ace_marker-layer .ace_bracket {
   margin: -1px 0 0 -1px;
-  border: 1px solid #cccccc;
+  border: 1px solid rgba(208, 135, 112, 0.4);
 }
 
 .ace_marker-layer .ace_active-line {
 .ace_bracket {
   margin: 0 !important;
   border: 0 !important;
-  background-color: #cccccc;
+  background-color: rgba(208, 135, 112, 0.4);
 }
 
 .ace_console_error {
 /* ---- RStudio Theme ---- */
 /* background */
 .rstudio-themes-flat .rstudio-themes-default {
-  background: #fafafa !important;
+  background: #f9fafb !important;
   color: #4c566a !important;
 }
 
 .rstudio-themes-flat .rstudio-themes-default .rstudio-themes-background {
-  background: #fafafa !important;
+  background: #f9fafb !important;
 }
 
 /* inactive tabs */
 }
 
 .rstudio-themes-flat .themedPopupPanel {
-  border: solid 1px #cccccc;
+  border: solid 1px rgba(208, 135, 112, 0.4);
 }
 
 .rstudio-themes-flat .themedPopupPanel ~ div.popupContent div:nth-child(1) {
-  border: solid 1px #cccccc;
+  border: solid 1px rgba(208, 135, 112, 0.4);
 }
 
 .rstudio-themes-flat .popupContent {
 
 .rstudio-themes-flat .popupContent:not(.popupMiddleCenterInner) div div:nth-child(2) {
   background: white !important;
-  border: solid 1px #cccccc;
+  border: solid 1px rgba(208, 135, 112, 0.4);
 }
 
 .rstudio-themes-flat .gwt-PopupPanel .popupContent {

rsthemes/solarized-light.rstheme

 
 .ace_marker-layer .ace_bracket {
   margin: -1px 0 0 -1px;
-  border: 1px solid #839496;
+  border: 1px solid rgba(220, 50, 47, 0.4);
 }
 
 .ace_marker-layer .ace_active-line {
 }
 
 .ace_indent-guide {
-  border-right: 1px solid #93a1a1;
+  border-right: 1px solid #eee8d5;
 }
 
 .ace_active-line ~ .ace_selection.ace_start {
 }
 
 .ace_invisible {
-  color: #839496;
+  color: #eee8d5;
 }
 
 .ace_variable {
 .ace_bracket {
   margin: 0 !important;
   border: 0 !important;
-  background-color: #839496;
+  background-color: rgba(220, 50, 47, 0.4);
 }
 
 .ace_console_error {
 }
 
 .rstudio-themes-flat .themedPopupPanel {
-  border: solid 1px #839496;
+  border: solid 1px rgba(220, 50, 47, 0.4);
 }
 
 .rstudio-themes-flat .themedPopupPanel ~ div.popupContent div:nth-child(1) {
-  border: solid 1px #839496;
+  border: solid 1px rgba(220, 50, 47, 0.4);
 }
 
 .rstudio-themes-flat .popupContent {
 
 .rstudio-themes-flat .popupContent:not(.popupMiddleCenterInner) div div:nth-child(2) {
   background: #fdf6e3 !important;
-  border: solid 1px #839496;
+  border: solid 1px rgba(220, 50, 47, 0.4);
 }
 
 .rstudio-themes-flat .gwt-PopupPanel .popupContent {

solarized-light.scss

 $ui-line-active-gutter    : $ui-gutter-color;
 $ui-line-active-selection : $base1;
 $ui-line-find             : $base00;
-$ui-bracket               : $base0;
-$ui-invisible             : $base0;
-$ui-indent-guide          : $base1;
+$ui-bracket               : transparentize($ui-cursor, 0.6);
+$ui-invisible             : $base2;
+$ui-indent-guide          : $base2;
 $ui-debug-background      : lighten($ui-background, 10%);
 $ui-fold-arrows-background: $violet;
 $ui-fold-arrows-color     : $ui-foreground;