Do a search in your CSS for the word “Forms” –
/* ## Forms
--------------------------------------------- */
Everything below that line (396 by default) handles some of the forms and their basic styling.
If you have a Plugin like Gravity Forms, Ninja, or any other “Form” plugin, then those plugins may override the default CSS. Mobile devices may or may not recognize the default CSS and may use their own styling to display things like checkboxes, list selection, etc. It just depends on the device.