(+ New topic) button in the header on all pages

To hide the button on mobile add

.header-buttons #new-create-topic {

in your mobile > CSS tab

The floating button you are referring is the one used on the Material Design Stock Theme component? If so you need to copy and paste the part of CSS relative to the floating button (New Topic Fab) from https://github.com/discourse/material-design-stock-theme/blob/master/desktop/desktop.scss to your mobile > CSS tab (deleting the lines that start with @include [...]).

However your requests are going off topic, it’s better if you open another topic.