Been using the Desktop add-on for years, but that has stopped working with Nightly and I had to move on since there were no updates.
I suggest you add the options to remove the box-shadow and background-fill. And also the option to hide the locked-toggle, maybe even the Settings button, as well.

For those that want to do it, in the meantime, you can do it through the following CSS code (look up how to use userContent.css):

@-moz-document url-prefix("moz-extension://f80a360e-8248-493f-9d3f-717b8bccc341/newTab.xhtml") {

/*Hide box-shadow and background-fill*/
.newtab-thumbnail {
box-shadow: none !important;
background-color: transparent !important;
}

/*Hide lock toggle*/
#locked-toggle {
display: none !important;
}
}

Thank you for the add-on. Keep it up.