fix night theme png downloads (#44941)

* include dark background to PNG exports when night theme is enabled

* fix css

* override border for png downloads

frontend/src/metabase/dashboard/components/DashCard/DashCard.styled.tsx

 import { css, type Theme } from "@emotion/react";
 import styled from "@emotion/styled";
 
+import { SAVING_DOM_IMAGE_CLASS } from "metabase/visualizations/lib/save-chart-image";
+
 export interface DashCardRootProps {
   isNightMode: boolean;
   isUsuallySlow: boolean;
@@ -38,6 +40,11 @@ export const DashCardRoot = styled.div<DashCardRootProps>`
 
   ${({ shouldForceHiddenBackground }) =>
     shouldForceHiddenBackground && hiddenBackgroundStyle}
+
+  &.${SAVING_DOM_IMAGE_CLASS} {
+    border-radius: 0;
+    border: none !important;
+  }
 `;
 
 export const VirtualDashCardOverlayRoot = styled.div`

frontend/src/metabase/dashboard/components/DashCard/DashCard.tsx

@@ -275,6 +275,7 @@ function DashCardInner({
     <ErrorBoundary>
       <DashCardRoot
         data-testid="dashcard"
+        data-dashcard-key={dashcard.id}
         className={cx(
           DashboardS.Card,
           EmbedFrameS.Card,

frontend/src/metabase/query_builder/actions/downloading.ts

@@ -43,9 +43,15 @@ export const downloadQueryResults =
     }
   };
 
-const downloadChart = async ({ question }: DownloadQueryResultsOpts) => {
+const downloadChart = async ({
+  question,
+  dashcardId,
+}: DownloadQueryResultsOpts) => {
   const fileName = getChartFileName(question);
-  const chartSelector = `[data-card-key='${getCardKey(question.id())}']`;
+  const chartSelector =
+    dashcardId != null
+      ? `[data-dashcard-key='${dashcardId}']`
+      : `[data-card-key='${getCardKey(question.id())}']`;
   await saveChartImage(chartSelector, fileName);
 };