UserContent.css

/* Fix new tab flash */
@-moz-document url-prefix(about:blank), url-prefix(about:newtab), url-prefix(about:home), url(moz-extension://e9518bb3-7ffc-4260-bdc8-c62be69fbc68/redirect.html), url(moz-extension://67af9bc8-7ba8-4477-9afa-924b0139681a/html/newtab.html)
{
    html:not(#ublock0-epicker), html:not(#ublock0-epicker) body, #newtab-customize-overlay
    {
        background: #202124 !important;
    }
}