blend colors against background instead of hard-coding to lighten

Author: cscheid

src/resources/formats/html/bootstrap/_bootstrap-variables.scss

@@ -234,12 +234,14 @@ $code-annotation-higlight-bg: #aaaaaa22 !default;
 $breadcrumb-divider: quote(">") !default;
 
 // table variable overrides
-$table-group-separator-color: lighten(
+$table-group-separator-color: mix(
   if(variable-exists(body-color), $body-color, $gray-900),
+  $body-contrast-bg,
   50%
 ) !default;
-$table-group-separator-color-lighter: lighten(
+$table-group-separator-color-lighter: mix(
   if(variable-exists(body-color), $body-color, $gray-900),
+  $body-contrast-bg,
   70%
 ) !default;
 
@@ -267,4 +269,8 @@ $font-weight-monospace-inline: $font-weight-monospace !default;
 $code-block-font-size: $code-font-size !default;
 $code-inline-font-size: $code-font-size !default;
 $link-weight: $font-weight-base !default;
-$link-decoration: inherit !default;
+$link-decoration: inherit !default;
+
+// border colors
+$border-color: mix($body-color, $body-contrast-bg, 30%) !default;
+$table-border-color: $border-color !default;