fix: color of hr elements in markdown

Fixes that the `hr` elements that were being generated in the markdown didn't match the rest of the app.

Author: crisbeto

report-app/src/styles.scss

@@ -67,6 +67,13 @@ html.dark-mode {
   box-sizing: border-box;
 }
 
+// These can show up in generated markdown.
+hr {
+  border: none;
+  background: none;
+  border-top: solid 1px var(--border-color);
+}
+
 @include buttons.button-styles;
 @include inputs.input-styles;
 @include callouts.callout-styles;