defaultTokens.tokens

<aura:tokens extends="force:base">
  <aura:token
    name="myYellow"
    value="#fcba03" />
  <aura:token
    name="colorWarning"
    value="{!colorBackgroundWarning}" />
  <aura:token
    name="colorBrand"
    value="{!colorBrand}" />
</aura:tokens>

designTokenDemo.css

div.box {
  width: 100%;
  height: 5rem;
}

div.box.warning {
  background-color: var(--lwc-colorWarning,#fc3000);
}

div.box.slds-warning {

}

div.box.brand {
  background-color: var(--lwc-myColorBrand,#7eeba2);
}

div.box.slds-brand {
  background-color: var(--lwc-colorBrand,#7eeba2);
}

designTokenDemo.html

<template>
  <div class="slds-grid slds-gutters slds-wrap">
    <div class="slds-col slds-size_1-of-3 slds-m-top_x-small">
      <div class="box warning">
        <div>lwc</div>
        custom warning, fall back red
      </div>
    </div>
    <div class="slds-col slds-size_1-of-3 slds-m-top_x-small">
      <div class="box warning">
        <div>lwc</div>
        custom warning, fall back red
      </div>
    </div>
    <div class="slds-col slds-size_1-of-3 slds-m-top_x-small">
      <div class="box warning">
        <div>lwc</div>
        Cannot save .css with "--lwc-colorBackgroundWarning", Internal Token, not Global Access
      </div>
    </div>
    <div class="slds-col slds-size_1-of-3 slds-m-top_x-small">
      <div class="box brand">
        <div>lwc</div>
        custom brand, fall back green
      </div>
    </div>
    <div class="slds-col slds-size_1-of-3 slds-m-top_x-small">
      <div class="box brand">
        <div>lwc</div>
        custom brand, fall back green
      </div>
    </div>
    <div class="slds-col slds-size_1-of-3 slds-m-top_x-small">
      <div class="box slds-brand">
        <div>lwc</div>
        slds-brand, fall back green
      </div>
    </div>
  </div>
</template>

TokenContainer.cmp

<aura:component description="TokenContainer"
  implements="flexipage:availableForAllPageTypes"
  access="global">

  <div class="aura-box">
    Aura, colorBackgroundWarning
  </div>

  <div class="slds-box">
    <div>Aura > LWC</div>
    <c:designTokenDemo />
  </div>
</aura:component>

TokenContainer.css

.THIS.aura-box {
  background-color: t(colorWarning);
  width: 100%;
  height: 5rem;
}

Screen Shot
https://prnt.sc/pym2yc

Experimenting with SLDS Design Tokens for LWC.

According to the docs, a single Aura component with Design Tokens must be in the page - does not need to be a parent of the LWC - in order for the LWC to use Design Tokens.

What I've found is that an Aura Component is not necessary to use Design Tokens in an LWC - however, custom design tokens are not supported, and ONLY Globally Accessible Tokens are available to LWC.

Design Tokens Docs:
https://developer.salesforce.com/docs/component-library/documentation/lwc/lwc.interop_components_css_aura