Consolidate SCSS variables for border colors

app/assets/stylesheets/framework/awards.scss

app/assets/stylesheets/framework/blocks.scss

app/assets/stylesheets/framework/buttons.scss

app/assets/stylesheets/framework/common.scss

app/assets/stylesheets/framework/sidebar.scss

app/assets/stylesheets/framework/typography.scss

app/assets/stylesheets/framework/variables.scss

app/assets/stylesheets/mailers/highlighted_diff_email.scss

...ssets/stylesheets/page_bundles/_ide_monaco_overrides.scss

...assets/stylesheets/page_bundles/_ide_theme_overrides.scss

app/assets/stylesheets/page_bundles/ide.scss

app/assets/stylesheets/page_bundles/milestone.scss

app/assets/stylesheets/page_bundles/pipelines.scss

app/assets/stylesheets/pages/commits.scss

app/assets/stylesheets/pages/groups.scss

app/assets/stylesheets/pages/issues.scss

app/assets/stylesheets/pages/notes.scss

app/assets/stylesheets/print.scss

app/assets/stylesheets/snippets.scss

app/assets/stylesheets/themes/_dark.scss