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;
}