Start on the affected page in a private browser window. Inspect the element that changed and expand the property in Computed styles. The browser lists the declaration responsible for the final value. Click its source link to see whether it came from a stylesheet, an inline style, or a style block in the document.
If the source is a bundled file, copy the selector and search for it in your theme, page builder settings, and custom CSS. For example, a button’s padding might come from a template rule even when the button itself has no custom settings. Inspect the selector and the surrounding rules before assigning ownership.
If you can reproduce the difference only at a certain screen width, check the media query around the winning declaration. Toggle the rule in developer tools to see whether removing it restores the intended layout. This change affects only your browser, so it is useful for testing a hypothesis before editing WordPress.
Record the URL, viewport width, selector, property, and source. Apply the smallest correction in the theme or editor setting that owns the rule. Then check the page in a fresh private window and test another page using the same template. That second page helps you catch unintended changes to shared styles.