Add expanding the environment variables in csspath

Custom CSS (Cascasding Style Sheets) can be passed on the command line using the --css option
In the current realization pytest-html checks existing of the css file with the next code

for csspath in config.getoption("css"):
    if not Path(csspath).exists():
        missing_css_files.append(csspath)

Сould you add the ability to use environment variables in the css file path? This has already been implemented for html path in HTMLReport class by os.path.expandvars

It would look like:

for csspath in config.getoption("css"):
    absolute_css_path = Path(os.path.expandvars(csspath)).expanduser().absolute()
    if not absolute_css_path.exists():
        missing_css_files.append(absolute_css_path)

Start by locating the css option handling and the existing os.path.expandvars usage in the HTMLReport class. Update the CSS path existence check consistently with that behavior, then verify that an environment-variable CSS path is accepted and missing expanded paths are reported correctly.