StickyHeader: Persist "darkMode"

[DmnChzl]

import { onMounted } from 'vue';

onMounted(() => {
  const item = sessionStorage.getItem('_read_write_app');

  if (item) {
    const { theme: themeValue = 'light' } = JSON.parse(item);
    if (themeValue === 'dark') document.documentElement.classList.add('dark');
    // TODO: theme.value = themeValue;
  }
});

[DmnChzl]

bugfix/persist-darkmode