Fix/1577/item card color contrast (#2921)

* fix(layout): adjust ItemCard header colors

- adjust header colors to pass color contrast standards

* chore(changeset): add changeset

-add change set

.changeset/heavy-berries-hear.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core': patch
+---
+
+Update ItemCard headers to pass color contrast standards.

packages/core/src/layout/ItemCard/ItemCard.tsx

@@ -20,8 +20,7 @@ const useStyles = makeStyles(theme => ({
   header: {
     color: theme.palette.common.white,
     padding: theme.spacing(2, 2, 6),
-    backgroundImage:
-      'linear-gradient(-137deg, rgb(25, 230, 140) 0%, rgb(29, 127, 110) 100%)',
+    backgroundImage: 'linear-gradient(-137deg,  #4BB8A5 0%,  #187656 100%)',
   },
   content: {
     padding: theme.spacing(2),