Block width

I know the block width is related to the theme. And somehow this resolves to 540 pixels for me - which is way too narrow.

How can I change the block width to 90%
(this is the width of the text inside the editor, I think it applies to both tinymce aka classic editor and the gutenberg editor).


I created a theme and was able to change the margin left hand side - but not the actual width.

add_filter( 'admin_footer', 'fulleditorwidth', 99 );
function fulleditorwidth() {
echo '<style> #content_ifr { display: block; margin-left: 3px; margin-right: 3px; width: 1900px !important; max-width:none !important;}</style>';
}