refactor: Replace white & black colors with theme aware ones

Signed-off-by: Carlos Esteban Lopez <lcarlosesteb@vmware.com>

plugins/explore/src/components/GroupsExplorerContent/GroupsDiagram.tsx

@@ -64,7 +64,7 @@ const useStyles = makeStyles((theme: BackstageTheme) => ({
     display: 'flex',
     alignItems: 'center',
     justifyContent: 'center',
-    color: 'black',
+    color: theme.palette.common.black,
   },
   legend: {
     position: 'absolute',

plugins/explore/src/components/ToolCard/ToolCard.tsx

@@ -41,7 +41,7 @@ const useStyles = makeStyles<BackstageTheme>(theme => ({
   },
   lifecycle: {
     lineHeight: '0.8em',
-    color: 'white',
+    color: theme.palette.common.white,
   },
   ga: {
     backgroundColor: theme.palette.status.ok,