you know tools like Firebug for Firefox? With these, you can easily see all CSS definitions for any HTML element and also see from which file the come from.
In the standard admin Layout, any content in the “main window” is wrapped in
<div class="ui main container basic segment very padded">
<div> has a standard padding of 3em causing the distance.
Change this to 0 and the IFrame should take all the space outside the menues.
If you do this directly in
semantic.css, you will have a hard time to update this file. IMO its better to create your own style sheet and overwrite the settings in there:
$app->requireCSS, you can easily load your own CSS file into the HTML output:
your_own.css, overwrite the settings:
Did the same here.