What?

Brave does this nice thing where depending if you're in an incognito window or not it changes the window's theme to represent it. I liked that so much that I made it into Firefox. I will also not be explaining every line of code here because it is dead simple to read and understand.

<aside> ❤️ Notable Help: u/qaz69wsx, u/black7375 & u/It_Was_The_Other_Guy

</aside>

:root[privatebrowsingmode="temporary"] {

    /* Toolbar */
    --toolbar-bgcolor: #322560 !important;

    /* Behind Tabs */
    --lwt-accent-color: #19162F !important;

    /* Search Bar Colour */
    --toolbar-field-background-color: #0B0724 !important;

    /* Turns Tab Into Same Colour As Toolbar */
    --lwt-selected-tab-background-color: var(--toolbar-bgcolor) !important;

    /* What Happens When You Hover Over A Button */
    --toolbarbutton-hover-background: rgba(255, 255, 255, 0.05) !important;

    /* Click On A Button, This Colour Is What The Button Will Be */
    --toolbarbutton-active-background: #4D4175 !important;

    /* Click A Button With A Context Menu? This Is The Menu Colour */
    --arrowpanel-background: #1D1935 !important;

    /* Tool Bar Context Menu Outline */
    --arrowpanel-border-color: #322560 !important;

    /* Line Between Toobar And Site */
    --chrome-content-separator-color: none !important;

    /* Colour Of The Expanded Address Bar */
    --toolbar-field-focus-background-color:var(--toolbar-field-background-color) !important;

    /* Hovering Over The Autocomplete Items Colour */
    --autocomplete-popup-hover-background: #1D1935 !important;

    /* Clicking On Said Autocomplete Item Colour */
    --autocomplete-popup-highlight-background: rgba(65, 48, 117, 0.5) !important;
}

/* Right Click Menu */
:root[privatebrowsingmode="temporary"] menupopup {
    --menuitem-hover-background-color: #413D54 !important;
    --menu-background-color: #1D1935 !important;
    --menu-color: #eeeeee !important;
    --menuitem-disabled-hover-background-color: #ffffff00 !important;
    --menu-disabled-color: #ffffff50 !important;
    --menu-border-color: #322560 !important;
}

/* Search Bar Popout Outline Killer */
#urlbar[open] > #urlbar-background {
    border-color: #ffffff00 !important;
}

/* No More Icon Incognito Icon */
.accessibility-indicator, .private-browsing-indicator{
    display: none !important;
}

/* URL Bar Fade Animation On Hover */
/* Thanks u/It_Was_The_Other_Guy   */
:root[privatebrowsingmode="temporary"] #urlbar-input-container:hover {
    background-color: #1F1B36 !important;
}