Use css variables to support adding a theme switcher

[vincentfretin]

Use css variables to support adding a theme switcher. (in a separate PR #835)
The variables names are those from https://daisyui.com so if your website is using daisyui, the inspector will be in your theme colors.

Example, if you have on your page

    <style>
      /* You can also create your theme with https://daisyui.com/theme-generator
       * The default theme will be in this case your generated theme and not the aframe default theme.
       */
      :root {
        color-scheme: light;
        --color-base-100: oklch(100% 0 0);
        --color-base-200: oklch(93% 0 0);
        --color-base-300: oklch(86% 0 0);
        --color-base-content: oklch(22.389% 0.031 278.072);
        --color-primary: oklch(58% 0.158 241.966);
        --color-primary-content: oklch(100% 0 0);
        --color-secondary: oklch(55% 0.046 257.417);
        --color-secondary-content: oklch(100% 0 0);
        --color-accent: oklch(60% 0.118 184.704);
        --color-accent-content: oklch(100% 0 0);
        --color-neutral: oklch(0% 0 0);
        --color-neutral-content: oklch(100% 0 0);
        --color-info: oklch(60% 0.126 221.723);
        --color-info-content: oklch(100% 0 0);
        --color-success: oklch(62% 0.194 149.214);
        --color-success-content: oklch(100% 0 0);
        --color-warning: oklch(85% 0.199 91.936);
        --color-warning-content: oklch(0% 0 0);
        --color-error: oklch(70% 0.191 22.216);
        --color-error-content: oklch(0% 0 0);
        --radius-selector: 0.25rem;
        --radius-field: 0.25rem;
        --radius-box: 0.25rem;
        --size-selector: 0.25rem;
        --size-field: 0.25rem;
        --border: 1px;
        --depth: 0;
        --noise: 0;
      }
    </style>

it will give