I2P_in_Private_Browsing_Mode_for_Firefox/background.js

browser.windows.onCreated.addListener(themeWindow);

// Theme all currently open windows
browser.windows.getAll().then(wins => wins.forEach(themeWindow));

function themeWindow(window) {
    // Check if the window is in private browsing
    if (window.incognito) {
            browser.theme.update(window.id, {
                images: {
                headerURL: "",
            },
            colors: {
                accentcolor: "#A0A0DE",
                textcolor: "white",
                toolbar: "#A0A0DE",
                toolbar_text: "white"
            }
        });
    }
    // Reset to the default theme otherwise
    else {
        browser.theme.update(window.id, {
            images: {
                headerURL: "",
            },
            colors: {
                accentcolor: "#BFA0DE",
                textcolor: "white",
                toolbar: "#BFA0DE",
                toolbar_text: "white"
            }
        });
    }
}