To change color of theme, you simply adding custom css. Create your own css file and then use:

App::create()
->css("/url/to/your/file.css")
...
->run();

So you can add some css rule to overwrite the header bar color.

For second issue, it is not a bug, rather it is the alternative text when image is not shown. Since that image supposes to be avatar but image url is not correct so you see the name there. To solve that issue, you provide a general user image.

That is the css style of the <img>, just provide the correct url for avatar, it will be solved.

Yes, you can use inspector tool of browser, for example, you can set header like this:

header {
    background-color:red !important;
}