Firefox Customs

hi, I would like to know how to Apply margin to web content but not when a video is in fullscreen? and keep the margin when Firefox is in fullscreen. I mean I would like to apply a margin to web content that not affect when a video is in fullscreen. I used this code:

:root:not([chromehidden~="toolbar"]) {
        
        /* Web content */
        & #appcontent{
            margin-inline-start: var(--my-vertical-toolbar-width) !important;
        }

        /* Sidebar + sidebar content */
        & #sidebar-box[checked="true"] {
            margin-inline-start: var(--my-vertical-toolbar-width) !important;
        }

        /* Sidebar + sidebar content + web content */
        & #sidebar-box[checked="true"] ~ #appcontent {
            margin-inline-start: 0px !important;
        }
    }

but that applies a margin when a video is in fullscreen, I tried too this code but removes the margin when Firefox is in fullscreen:

:root:not([chromehidden~="toolbar"],[sizemode="fullscreen"]) {
     ...
}