[011y Overview] Align and review CTA buttons in all header areas (elastic#229437)

Closes elastic#229281 

## Summary

This PR changes the styles of the refresh button as described in the
issue

| Before | After |
| ------ | ------- |
| <img width="2568" height="1254" alt="image"
src="https://github.com/user-attachments/assets/2c791b89-8629-4b9e-9d1d-976a2d1731af"
/>| <img width="3810" height="1526" alt="image"
src="https://github.com/user-attachments/assets/d9312a8e-b390-43c8-82b4-dbabce6dd0c2"
/> |

Author: jennypavlova

x-pack/solutions/observability/plugins/observability/public/pages/overview/components/date_picker/date_picker.tsx

@@ -76,6 +76,7 @@ export function DatePicker({
       onRefreshChange={onRefreshChange}
       commonlyUsedRanges={commonlyUsedRanges}
       width={width}
+      updateButtonProps={{ fill: false }}
     />
   );
 }