Floating left

How can I make the green bar

It’s not a green bar: it’s a green and white bar.

So the white to the left you’re seeing is not due to float, alignment, margin, padding or any sort of spacing. The white is part of the image itself.

Edit your image to remove the white part.

Editing the image as George suggests will work. A different solution would be to replace the image block with a div element styled to have a fixed height and to fill 100% of its container’s width. The div’s background color would be the green color you want.

Even though your image file is tiny in size, it’s generally preferable where possible to implement graphic elements with pure HTML/CSS instead of external image files.