Created attachment 9118 
Image of the way borders work before and after the CSS code provided in the initial report.

This is purely cosmetic and trivial but the frame borders around the text view are extra thick, and more so when there are tabs. 

When no tabs are present the left and right borders are doubled, they press against the window border giving it a double border look.

When tabs are present the top and bottom borders are doubled and the left and right borders are tripled.

I'll add a picture to show what I mean. I have managed to fix this with a custom CSS. Mousepad lacks uniquely named CSS style classes so while this works for Mousepad, it messes up many other apps. As a secondary request might MousepadWindow be given a style class name unique to mousepad? Thunar, for example, I can add .thunar before the CSS style and it will only affect Thunar.

Anyway, here is the CSS code I used below. I tested it on my own theme, Adwaita, and Greybird and it seems to work. Maybe it will be of some help so I'll add it.

Again this is super trivial and just an enhancement request. Thanks for the amazing editor!

-------------------------
--------- CSS ---------
-------------------------

/* Hides left and right frame when there are no tabs */

notebook scrolledwindow.frame
{
    margin-left: -1px;
    margin-right: -1px;
}

/* Hides the left border of the first tab when against the window edge */

notebook tabs
{
    margin-left: -1px;
}

/* Removes top, left, right borders when there are tabs */
/* Bottom border moved down so it hides the notebook bottom border */

notebook stack .frame:not(:only-child)
{
    margin-bottom: -1px;
    border-top: none;
    border-left: none;
    border-right: none;
}

/* This is added so above code doesn't affect dialog box */

.dialog-vbox notebook tabs
{
margin-left: 0;
}

/*  Just added some padding between the tab text and close button */

notebook header.top tab button:only-child
{
    margin-left: 5px;
    margin-right: 0;
}