The cleanest way to modify the CSS
of an existing theme is to leave it's files untouched but add your modifications in a separate file.
1) So put the following CSS in a file called "osaka.css":
2) I suppose you are using the "Standard Logo" theme, so put that file in the theme folder.
3) Open the config.php in that folder and add the name of your CSS file (without the .css extension):
$THEME->sheets = array('gradients', 'osaka');
I separated login info and language chooser so you can position them separately so that they fit nicely around the green line of your logo.
For finetuning the positions use Firebug
For an even cleaner separation of your own code and the standard themes you should create your own theme by using one of the existing ones as parent theme (e.g. "Standard Logo").
See How do I create a custom theme?
for further information.