BugWP logo BugWP
Back to Community
Home / Community / Theme → Twenty twelve

Troubleshooting Twenty Twelve Child Theme Responsiveness Issues

33 threads Sep 16, 2025 Theme → Twenty twelve

Content

Many users of the classic Twenty Twelve theme rely on child themes to safely make customizations. A common and frustrating issue that arises is when a previously well-functioning child theme suddenly becomes unresponsive on mobile devices. This article will explain why this happens and guide you through the most effective solutions.

The Core Problem: Outdated PHP and CSS Conflicts

Based on community reports, the primary culprits for a Twenty Twelve child theme losing its responsiveness are:

  1. An Outdated PHP Version: This is a frequent cause. As seen in user reports, a child theme that is responsive under PHP 5.5 may break when the hosting environment is upgraded to PHP 5.6, 7.x, or 8.x. The parent Twenty Twelve theme itself is compatible with modern PHP versions (including 8.1), so the issue almost always lies within custom code in the child theme's functions.php or other template files that contains deprecated functions or syntax errors. These errors can halt CSS processing, breaking the responsive design.
  2. CSS Overrides in the Child Theme: Custom CSS rules in the child theme's style.css file can inadvertently override or conflict with the parent theme's mobile-responsive media queries. A simple float, margin, or width declaration can disrupt the carefully designed fluid layout.

How to Diagnose and Fix the Issue

Follow these steps to identify and resolve the problem.

Step 1: Isolate the Cause

First, you need to figure out if the problem is due to PHP or CSS.

  • Switch to the Parent Theme: Temporarily activate the parent Twenty Twelve theme. If the site is now responsive, you have confirmed the issue is isolated to your child theme.
  • Check Your PHP Version and Error Logs: Contact your web host to confirm which PHP version your site is running. Enable debugging (temporarily) by adding the following lines to your child theme's functions.php file or your site's wp-config.php file. This will help surface any hidden PHP errors that are causing the white screen or broken layout. 
    define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true ); // Logs errors to wp-content/debug.log
define( 'WP_DEBUG_DISPLAY', false ); // Hides errors from the screen

Step 2: Fix PHP Compatibility Issues

If debugging reveals errors in your debug.log file, your custom PHP code needs to be updated for compatibility with newer PHP versions. Common fixes include:

  • Replacing deprecated functions with their modern equivalents.
  • Ensuring all code follows proper syntax (e.g., no call-time pass-by-reference, which was a fatal error in one user's plugin).
  • If you are not comfortable editing PHP, consider seeking help from a developer familiar with WordPress and PHP modernization.

Step 3: Review and Fix Child Theme CSS

If PHP is not the issue, the problem is almost certainly in your CSS. The solution is to methodically review your customizations.

  • Revert to a Basic Child Theme: The quickest test is to temporarily replace your child theme's style.css with a minimal version. A basic child theme should only contain the required header and an import of the parent's stylesheet: 
    /*
Theme Name:   Twenty Twelve Child
Template:     twentytwelve
*/

@import url("../twentytwelve/style.css");
    If the site is responsive with this basic CSS, you know your original custom CSS contains the problem.
  • Re-add Custom CSS Gradually: Slowly add your custom CSS rules back into the style.css file, checking the mobile responsiveness after each addition. This will help you pinpoint the exact rule causing the conflict.
  • Inspect Media Queries: Use your browser's developer tools (F12) to inspect the mobile layout. Check if the parent theme's media queries are being applied and if your custom CSS is overriding any critical properties like max-width or float within those queries.

Conclusion

A non-responsive Twenty Twelve child theme is a classic case of custom code colliding with modern standards. The process of fixing it is a systematic one: confirm the child theme is the source, check for PHP errors, and meticulously audit your CSS. By following these troubleshooting steps, you can restore your site's mobile-friendly design and ensure it remains compatible for the future.

Have you encountered and solved this issue with a different method? Share your experience in the comments below to help other users.

Related Support Threads Support