arbitrary add_css_file attributes are not supported

[gmarull]

Problem

It is sometimes necessary to add a CSS file with attributes such as media (for print, dark mode, etc.). As of today, this is not possible when using sphinx_rtd_theme because attributes seem to be ignored (excepting title): https://github.com/readthedocs/sphinx_rtd_theme/blob/master/sphinx_rtd_theme/layout.html#L27

Reproducible Project

Create a custom CSS file and add it to your project like this:

app.add_css_file("css/my_customization.css", media="print")

In the example above, media attribute is ignored on the rendered pages.

Error Logs/Results

N/A

Expected Results

All attributes given to CSS files should be taken into account.

Environment Info

• Python Version: 3.8.5
• Sphinx Version: 3.5.3
• RTD Theme Version: 0.5.1

[humitos]

Done in #1519