feat(DuoChat): consolidate sass variable for duo scrim

mentioned in commit 7b50864d

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1212 (closed)

mentioned in merge request status-page!903

mentioned in merge request gitlab!170923 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!55 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!352 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4159 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1249 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)