https://examples.sencha.com/extjs/7....c#grouped-grid
Снимок экрана 2019-09-23 в 11.36.16.jpg
As a workaround I set top/bottom padding of `x-grid-group-hd` to 10px;
Code:
.x-grid-group-hd {
    /*padding : 8px 10px;*/
    padding : 10px;
}
Снимок экрана 2019-09-23 в 11.44.20.jpg

Also I would recommend you to get rid of the black bottom border to match row styles and make the background color a bit lighter to match header background color. This will help to distinguish normal rows and group rows.
Code:
.x-grid-group-hd {
    /*padding : 8px 10px;*/
    padding : 10px;
    border-color : #6d6d6d;
    background: #6d6d6d;
}
Please compare the following screenshots:
#1 Default colors
2019-09-23_1201.jpg
#2 Adjusted colors, groups expanded
2019-09-23_1202.jpg
#3 Adjusted colors, groups collapsed
2019-09-23_1203.jpg