I don’t know how it looked on the Agama Blue theme, but seems to me it looks just fine.https://prnt.sc/tepdqv
If you want to change the shape color, Insert the following code into Customizing->General->Additional CSS:
.wp-custom-header svg .shape-fill {
fill: lightblue!important;
}
Anyway, If you want to change the height and width of the shape, you can use the following code(insert it in the custom CSS too) then change the width and height as you need.
.wp-custom-header svg{
height:50px!important;
width:100%!important
}
Regards