Current `sphinx_rtd_theme` doesn't support Sphinx>=4.0

**Is your feature request related to a problem? Please describe.**
Current html theme package `sphinx_rtd_theme` doesn't support Sphinx version >=4.0.
If sphinx version >= 4.0 is installed, rendered html doesn't have css style.

**Describe the solution you'd like**
Upgrade Sphinx to a version >=4.0 and find a compatible theme.

**Describe alternatives you've considered**
None.

**Additional context**
Wrong rendered html:

Correct rendered html:

Start by locating the documentation build configuration and its Sphinx and sphinx_rtd_theme dependencies. Reproduce the build with Sphinx >=4.0, identify a compatible theme, and verify that the rendered HTML includes the expected CSS styling.

Written by the indexing model from the issue text.