fix: card thumbnails now always show border. (#11638)

Co-Authored-By: Evan Rusackas <evan@preset.io>

superset-frontend/src/components/ListViewCard/ImageLoader.tsx

@@ -30,8 +30,9 @@ const ImageContainer = styled.div<ImageContainerProps>`
   background-size: cover;
   background-position: center ${({ position }) => position};
   display: inline-block;
-  height: 100%;
-  width: 100%;
+  height: calc(100% - 1px);
+  width: calc(100% - 2px);
+  margin: 1px 1px 0 1px;
 `;
 interface ImageLoaderProps
   extends React.DetailedHTMLProps<

superset-frontend/src/components/ListViewCard/index.tsx

@@ -80,6 +80,7 @@ const StyledCard = styled(Card)`
 
 const Cover = styled.div`
   height: 264px;
+  border-bottom: 1px solid ${({ theme }) => theme.colors.grayscale.light2};
   overflow: hidden;
 
   .cover-footer {