If you are using a classic MyBB theme from 2012 (e.g., Acid Orange , Default MyBB 1.6 ), it is almost certainly responsive. Attempting to force one with CSS media queries often breaks the complex table-based layouts of older templates. You are better off starting fresh with a modern, responsive framework.
In 2024, if your MyBB forum isn't mobile-friendly, it’s essentially invisible to a massive chunk of the web. Google prioritizes mobile-first indexing, and users expect seamless browsing—whether they’re on a 34-inch monitor, an iPad, or a budget Android phone. responsive mybb themes