Search bar: submit button and input field won't line up

Hello everyone,

I am new to this forum. I installed the “YOKO” theme on my wordpress blog and wanted to “customize” the search bar button. I changed the border radius and color, but now for some reason the submit button and input field won’t aline anymore: I get the input field in the first line and the line beneath contains the submit button.
Here is what my CSS stylsheet looks like:

[CSS moderated as per the Forum Rules. Please just post a link to your site.]

And this is what the function sections looks like:

* Search form custom styling
 */
function yoko_search_form( $form ) {
    $form = '<form role="search" method="get" class="searchform" action="'.get_bloginfo('url').'" >
    <div><label class="screen-reader-text" for="s">' . __('') . '</label>
    <input type="text" class="search-input" value="' . get_search_query() . '" name="s" id="s" />
   <input type="submit" class="searchsubmit" value="'. esc_attr__('Go!', 'yoko') .'" />
    </div>
    </form>';
    return $form;
}
add_filter( 'get_search_form', 'yoko_search_form' );

What am I doing wrong? I have tried to google the topic but can’t find any answers to my problem.
If anyone could help me it would be very much appreciated!
Thank you!