Hey there alyona,
If you are referring to this part https://screencast.com/t/YMyTSrx8Bnt. This in an image which can be changed in Appearance >> Header. You can upload your own image.
If you are referring to this part https://screencast.com/t/UtllNR7x it can be changed by adding some custom CSS. The easiest way to do that, if your theme doesn’t already have custom CSS tab, would be to add it in your site using the following plugin
https://www.ads-software.com/plugins/simple-custom-css
Once the plugin is installed and activated you can add this code to Appearance >> Custom CSS:
.navbar-inverse {
background-color: #222;
}
Replace the color hex value to the color of your choice.
Hope this helps ??
Best regards,
Bojan