Remove a Style Tag from WP Theme

The <html> tag is probably written in your theme’s header.php file, but you should avoid editing it directly as any change you make will be overwritten if the theme is ever updated.

The specific part you are asking to remove is CSS, but you don’t need to edit the theme to make CSS changes. Instead you can put your own CSS in Appearance > Customize > Additional CSS.

To override this particular CSS, use this:

html {
    overflow: visible !important;
}

visible is the default for this property, so setting it to that is the same as removing it. The !important is required as it’s the only way to override an inline rule according to the Specificity rules in CSS.