@-moz-document url-prefix("https://app.ringcentral.com/") {
body {
    filter: invert(1) hue-rotate(180deg) !important;
}

[data-test-automation-id="imageCard"] {
    filter: invert(1) hue-rotate(160deg) !important;
}
}

/* ==UserStyle==
@name        Ringcentral Dark Theme
@namespace   mikemanger
@version     1.0.0
@homepageURL https://github.com/mikemanger/userscripts/
@updateURL   https://raw.githubusercontent.com/mikemanger/userscripts/master/ringcentral-dark.user.css
@license     CC-BY-4.0
==/UserStyle== */

@-moz-document url-prefix("https://app.ringcentral.com/") {
body {
    filter: invert(1) hue-rotate(160deg) !important;
}

[data-test-automation-id="imageCard"] {
    filter: invert(1) hue-rotate(160deg) !important;
}
}

Add meta to ringcentral theme
Add meta to ringcentral theme