ASP.NET – cannot style forms

I have a problem applying CSS to <form> element and all nested elements inside. I do it by class .header_navbar_form and .header_navbar_form_input in stylesheet header.css, but only bootstrap takes effect. For some reason, other elements, such as <div>, <p>, <h1> are affected by the same stylesheet
Once again: div with id header_navbar_div is affected by the same stylesheet

Bootstrap 4.5 Dropdown appearing as multiple links on iOS

When I try to load a dropdown list in a Bootstrap navbar on an ASP.NET page, the desktop version of the list loads exactly as it appears on the official navbar documentation, but not when loading the same page on an iPad Air 2 with iOS 13.3.1. Instead of loading the listed items in a dropdown fashion, they appear below the dropdown menu’s name. Just as well, the toggle “hamburger” button appears as a blank button.
Bootstrap 4.5 Dropdown appearing as multiple links on iOS