This is not the right way to go. You should use a guided landing page template, not the old free form one with absolute positioning and terrible mobile capabilities.
In Marketo, as a global rule (and there are some exceptions) CSS should be added at template level, not at LP level.
If you must - publish the page, visit the page and open your browser's inspector tool to find the class or id for the elements you want to change. In FF, Chrome and IE, you can right click anywhere on a page and click "Inspect".
If you're more advanced, you can drag a html wysiwyg element onto the landing page canvas and write all of your code in there including setting your own IDs, classes, CSS and scripts. If you're confident in trying this, just letting you know there are default CSS settings you'll need to overwrite to make the html container style the way you want.