Update Theme: trackpad animation

[Fury7425]

What

Square off the page corners when the browser is in fullscreen so the page
background no longer bleeds through the rounded corners (most visible on
fullscreen video).

Why

.browserStack browser was given border-radius: var(--mod-browser-radius-default)
in every state, including fullscreen. In fullscreen there's no chrome behind the
page, so the rounded corners exposed the window background as a colored
outline/artifact at each corner.

How

Override the radius variable to 0 while fullscreen instead of touching the
rule selectors directly:

:root[sizemode="fullscreen"],
:root[inDOMFullscreen="true"] {
  --mod-browser-radius-default: 0px;
}