trilium/packages/share-theme/src/scripts/modules/theme.ts

const preference = localStorage.getItem("theme");
if (preference) {
    if (preference === "dark") {
        document.body.classList.add("theme-dark");
        document.body.classList.remove("theme-light");
    }
    else {
        document.body.classList.remove("theme-dark");
        document.body.classList.add("theme-light");
    }
}

export default function setupThemeSelector() {
    const themeSwitch: HTMLInputElement = document.querySelector(".theme-selection input")!;
    
    if (preference) {
        const themeSelection: HTMLDivElement = document.querySelector(".theme-selection")!;
        themeSelection.classList.add("no-transition");
        themeSwitch.checked = preference === "dark";
        setTimeout(() => themeSelection.classList.remove("no-transition"), 400);
    }

    themeSwitch?.addEventListener("change", () => {
        if (themeSwitch.checked) {
            document.body.classList.add("theme-dark");
            document.body.classList.remove("theme-light");
            localStorage.setItem("theme", "dark");
        }
        else {
            document.body.classList.remove("theme-dark");
            document.body.classList.add("theme-light");
            localStorage.setItem("theme", "light");
        }
    });
}