Javascript:

const themeButton = document.getElementById('theme-button')
const darkTheme = 'dark-theme'
const iconTheme = 'uil-sun'

// Previously selected topic (if user selected)
const selectedTheme = localStorage.getItem('selected-theme')
const selectedIcon = localStorage.getItem('selected-icon')

// We obtain the current theme that the interface has by validating the dark-theme class
const getCurrentTheme = () => document.body.classList.contains(darkTheme) ? 'dark' : 'light'
const getCurrentIcon = () => themeButton.classList.contains(iconTheme) ? 'uil-moon' : 'uil-sun'

// We validate if the user previously chose a topic
if (selectedTheme) {
  // If the validation is fulfilled, we ask what the issue was to know if we activated or deactivated the dark
  document.body.classList[selectedTheme === 'dark' ? 'add' : 'remove'](darkTheme)
  themeButton.classList[selectedIcon === 'uil-moon' ? 'add' : 'remove'](iconTheme)
}

// Activate / deactivate the theme manually with the button
themeButton.addEventListener('click', () => {
    // Add or remove the dark / icon theme
    document.body.classList.toggle(darkTheme)
    themeButton.classList.toggle(iconTheme)
    // We save the theme and the current icon that the user chose
    localStorage.setItem('selected-theme', getCurrentTheme())
    localStorage.setItem('selected-icon', getCurrentIcon())
})

CSS
.body.dark-theme{
      /* HSL color mode */
      --first-color-second: hsl(var(--hue-color), 30%, 8%);
      --title-color: hsl(var(--hue-color), 8%, 95%);
      --text-color: hsl(var(--hue-color), 8%, 75%);
      --input-color: hsl(var(--hue-color), 29%, 16%);
      --body-color: hsl(var(--hue-color), 28%, 12%);
      --container-color: hsl(var(--hue-color),29%, 16%);
}

/*========== Button Dark/Light ==========*/
.nav__btns{
  display: flex;
  align-items: center;
}

.change-theme{
  position: absolute;
  right: 1rem;
  top: 1.8rem;
  color: var(--text-color);
  font-size: 1rem;
  cursor: pointer;
}

.change-theme:hover{
  color: var(--first-color-alt);
}

HTML
 <!--Theme change button-->
                <i class="uil uil-moon change-theme" id="theme-button"></i>