To put CSS into a glide screen you need to add a Rich Text component to the screen or tab.
In that component, you paste in the CSS code.
For instance, if I put this into a Rich Text component, the CSS will remove the top bar and the tabs at the bottom of the screen:
<pre><span><style>
[id="tabBar"]{
display: none;
}
[data-test="glide-app-bar"] :nth-child(1) {
display: none;
}
#app-root div[opacity='1'] {
color: transparent;
}