Fix light mode colour token in ItemCardHeader (#31203)

The text colour in the ItemCardHeader component is always white, which makes the content unreadable that when you're rendering on a white background in light mode. This switches to a semantic token instead that will dynamically adjust to whichever theme is currently set.

Signed-off-by: James Brooks <jamesbrooks@spotify.com>

.changeset/fast-queens-guess.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core-components': patch
+---
+
+Swap base token for semantic token in ItemCardHeader to ensure readability in light mode.

packages/core-components/src/layout/ItemCard/ItemCardHeader.tsx

@@ -30,7 +30,7 @@ export type ItemCardHeaderClassKey = 'root';
 const styles = (theme: Theme) =>
   createStyles({
     root: {
-      color: theme.palette.common.white,
+      color: theme.palette.text.primary,
       padding: theme.spacing(2, 2, 3),
       backgroundImage: theme.getPageTheme({ themeId: 'card' }).backgroundImage,
       backgroundPosition: 0,