Sorry for all the questions!
I just changed my header background color, and it has two black boxes at the end.
It also happens on the live server! https://if-charts.000webhostapp.com
The CSS for the header is here:
https://github.com/IF-Apps/IF-Charts/blob/master/css/styles.css
And the file I showed you is here:
https://github.com/IF-Apps/IF-Charts/blob/master/index.html
I’m not sure if there may be something else affecting the header.
Sorry for the stupid questions, and thanks in advance!