Hi,
You have two ways to edit that.
In case you would like to change the css you simply can right-click the element, click on inspect and find the css selectors you would like to change. Next add the modified code tou the custom css of you website.
When you want to add elements like a logo or image go to Acymailing->Custom fields and create a field of the type custom text and put your htmal in that field. Next add the custom field to your subscription form.
