Thank you so much @AnneLuo . Error fixed 
Topic summary
A user encountered a display issue with the Ella theme where the “Sort By” dropdown menu was being overlapped by product images below it, preventing proper visibility.
Solution Provided:
- Add custom CSS code to the theme.liquid file before the
</head>tag - Access via: Online Store → Theme → Edit Code
- The CSS adjustment modifies the z-index property to ensure the dropdown appears above other elements
Outcome:
The issue was successfully resolved. The original poster confirmed the fix worked and thanked the contributor who provided the solution.