[Documentation] Better documentation of light/dark theme transition

[leembierman]

I am a product manager and design lead for a design team using Material as a framework for group of applications that will require both light and dark mode.

We are attempting to give the designers some guidelines for how material applies default.background and default.paper and how those colors transition when switching from light to dark and vice versa. Material-UI's Theme page (https://material-ui.com/customization/default-theme/) displays the hex values for light mode, but does not display the corresponding hex codes for dark mode when toggling to the dark theme.

The original attempt to remedy this issue was by using a color picker tool to "test" colors in light and dark mode while viewing them on www.material-ui.com. We also used the theme console command to determine what the theme properties were on www.material-ui.com. It quickly became apparent that Material-UI was not applying the default material theme to their own documentation website:

Light mode default.background color in the default theme is #FAFAFA, www.material-ui.com uses #FFF as their default.background; furthermore, www.material-ui.com uses two additional levels that are not found in the Material-UI theme: default.level1 and default.level2.

Some clarity on how Material-UI's default theme applies color to components in light and dark mode would help us build an accurate design library and ensure that toggling between light and dark mode in applications is seamless.

[m4theushw]

I'll work on a PR to make the inspector in the "Default Theme" page aware of the current theme.