Александр

+ function applyTheme() {
+     const isDark = (localStorage.getItem('is-dark-theme') ?? 'true') === 'true';
+     if (isDark) {
+         document.body.classList.remove('light-mode');
+         document.body.classList.add('dark-mode');
+     } else {
+         document.body.classList.remove('dark-mode');
+         document.body.classList.add('light-mode');
+     }
+ }

- var dark = true;
function themeChange() {
-     var element = document.body;
- 
-     if (dark == true) {
-         element.classList.remove("dark-mode");
-         element.classList.add("light-mode");
-         dark = false;
-     }
- 
-     else {
-         element.classList.remove("light-mode");
-         element.classList.add("dark-mode");
-         dark = true;
-     }
+     localStorage.setItem('is-dark-theme', !((localStorage.getItem('is-dark-theme') ?? 'true') === 'true'));
+     applyTheme();
}

+ window.addEventListener('DOMContentLoaded', function () {
+     applyTheme();
+ });

.event {
  width: 300px;
  height: 100px;
  
  &__caption {
    color: blue;
  }

  &__description {
    color: green;
  }
}

@media screen and (max-width: 768px) {
    .event {
        &__caption {
            color: red;
        }
    }
}

.event {
  width: 300px;
  height: 100px;
  
  &__caption {
    color: blue;
    
    @media screen and (max-width: 768px) {
      color: red;
    }
  }

  &__description {
    color: green;
  }
}