You installed this plugin:
which is adding this line of CSS:
* {
box-sizing: border-box;
}
The “*” is a wildcard selector that applies that style to everything on a webpage. A plugin should never do that. Targeting everything, instead of just the items the plugin affects, is just lazy coding, in my opinion. (I know it’s the plugin author’s attempts to make it mobile, but there really are better ways than this.)
Moved to Support.