Hi there, If you have not yet done so, I would suggest creating a child theme so that any customizations you make will not be overwritten by a future theme update. Alternately you can use the Edit CSS function in the Jetpack plugin, or use one of the other Custom CSS plugins that will keep CSS changes from being overwritten.
Jetpack plugin
Child Themes
Child Theme creation plugins
Custom CSS plugins
On Superhero, the header image sets to the left of the menu and the recommended size is 480px x 150px. Is that the image you are talking about, or are you talking about the full-width slider that would be below the black band containing the menu?
I see you have two flags on the right of the menu area, but the flags are pretty small. You can have a larger flag by placing the flag as a background image in li.sparch, but you will need to have a separate class for each flag, something like sparch-de, sparch-us, or similar and then add the background declaration to those classes, something like this:
li.sprach-us {
float: right;
display: block;
width: 50px;
background: url('http://www.andisaitenhieb.de/material/logo-british-flag300x150.png') no-repeat scroll center center;
background-size: contain;
}The background-size:contain; declaration will make the flag image fit the available space.
The yellow line at the bottom of the widgets is a bottom border declared in .widget. To get rid of them, you can do the following:
.widget {
border-bottom: none;
}Superhero does not have a footer widget area, but you can add Widget Areas to any theme. Again, this is best done using a child theme so that your customizations are not overwritten.