Enable dark mode viewing on doxygen documentation page

**Describe the problem you have/What new integration you would like**

esphome should have dark mode

**Please describe your use case for this integration and alternatives you've tried:**

As a user who frequently works on esphome at night,
I would like the class documentation (doxygen) to include an optional dark mode,
So that my eyes are not stained while looking at class documentation.

**Additional context**

Enabling darkmode can be enabled by adding an additional css style sheet

https://github.com/MaJerle/doxygen-dark-theme

And then modifying `HTML_EXTRA_STYLESHEET`

Start by reviewing the linked doxygen-dark-theme stylesheet and locate the project configuration that controls HTML_EXTRA_STYLESHEET. Enable the additional stylesheet for the Doxygen output and verify that class documentation offers the requested optional dark-mode viewing.

Written by the indexing model from the issue text.