The documentation shows it will dim all of its children:

<Dimmer active={true} page>
  <ChildComponent/>
</Dimmer>

Which will then dim the child component. Along with that you can also specify properties like "page" to tell it to dim the whole page.

If you're trying to use it with sidebar, then you have to wrap the content for sidebar in a dimmer.

const Page = () => {
  const [active, setActive] = useState(false)

  return <div>
    <Sidebar setActive={setActive}/>
    <Dimmer active={active} page> // this is a child
      <Content/>
    </Dimmer>
  </div>
}

Then in the sidebar you'll have to set the active when you click an item or whatever. This example isn't perfect, just gives you a rough idea of what you can do.

Alternatively:

.dimmed {
  background: black;
  opacity: 0.5;
}

You can also just do it yourself and apply a class with opacity to whatever needs to be dimmed.

CREDITS ON @Matriarx for giving me idea on how to answer his question...

You have to import the Dimmer on sematic-ui-react and then parent the child using this code:

 <Dimmer.Dimmable dimmed={toggleCart}>
        <Dimmer
          className="dimmedContent"
          active={toggleCart}
          onClickOutside={handleHide}
        />

put this on your css so that you can give yourself a style, without this css you cant make the dimmed fullscreen

.dimmedContent {
  display: block !important;
  position: fixed !important;
  top: 0 !important;
  left: 0 !important;
  right: 0 !important;
  bottom: 0 !important;
  background: rgba(0, 0, 0, 0.25) !important;
  cursor: default !important;
  z-index: 8 !important;
}

this is the full code:

import React, { useState } from 'react';
import { Dimmer } from 'semantic-ui-react';
import CartSidebar from './CartSidebar';
import Navbar from './Navbar';

const StoreLayout = ({ children }) => {
  const [toggleCart, setToggleCart] = useState(false);
  function toggleMenuCart() {
    setToggleCart(!toggleCart);
  }
  const handleHide = () => {
    setToggleCart({ active: false });
  };
  return (
    <>
      <Dimmer.Dimmable dimmed={toggleCart}>
        <Dimmer
          className="dimmedContent"
          active={toggleCart}
          onClickOutside={handleHide}
        />
      <CartSidebar toggleMenu={toggleCart} />
      <Navbar onToggleMenuCart={toggleMenuCart} />
      {children}
      </Dimmer.Dimmable>
    </>
  );
};

export default StoreLayout;

also dont forget to put the css that i provided above :)

Cheers!