docs: tune Doxygen theme contrast

[voltjia]

Summary

• Tune docs/assets/infini-rt.css for Doxygen Awesome instead of the default Doxygen theme.
• Improve contrast for the top project title, project brief, top navigation, and left tree navigation.
• Adjust inline code and code block colors so they remain readable with the current generated theme.

Motivation

The generated documentation currently renders with low-contrast left navigation and an unreadable project title on the dark header when the browser uses the Doxygen Awesome dark-mode variables. This PR keeps the Doxygen Awesome layout while making the visible navigation and inline code readable.

N/A - no linked issue.

Type of Change

• feat — new feature / new operator / new platform
• fix — bug fix
• perf — performance improvement (no behavioral change)
• refactor — code restructuring without behavior change
• test — adding or fixing tests only
• docs — documentation only
• build / ci — build system or CI configuration
• chore — tooling, formatting, or other non-code changes
• Breaking change (requires a ! in the Conventional Commits prefix or a BREAKING CHANGE: footer)

Platforms Affected

• CPU (WITH_CPU)
• NVIDIA (WITH_NVIDIA)
• Iluvatar (WITH_ILUVATAR)
• MetaX (WITH_METAX)
• Cambricon (WITH_CAMBRICON)
• Moore (WITH_MOORE)
• Ascend (WITH_ASCEND)
• PyTorch C++ bindings (WITH_TORCH)
• Build system / CMake / CI
• Python bindings / user-facing API

Smoke Test Result

$ git diff --check
# no output

Rendered locally from generated Doxygen Awesome HTML at http://127.0.0.1:8787/ and verified:

project title: rgb(245, 247, 251)
project brief: rgb(170, 182, 198)
left nav text: rgb(51, 65, 85)
selected nav: rgb(219, 234, 254)
inline code: rgb(23, 32, 51) on rgb(238, 243, 248)

I also attempted a fresh remote Doxygen rebuild on nvidia with accelerator-dev/nvidia:doxygen; that run was blocked by a transient GitHub FetchContent download error for the already-merged Doxygen Awesome tarball (Error in the HTTP2 framing layer). The visual check above used a real generated Doxygen Awesome HTML output with this PR's CSS loaded.

Test Results on Supported Platforms

Platform	Affected	Build / Smoke Result	Full Result / Notes
NVIDIA	No	N/A - docs-only CSS change	N/A - not affected
Iluvatar	No	N/A - not affected	N/A - not affected
MetaX	No	N/A - not affected	N/A - not affected
Cambricon	No	N/A - not affected	N/A - not affected
Moore	No	N/A - not affected	N/A - not affected
Ascend	No	N/A - not affected	N/A - not affected

Full `pytest` output (optional)

N/A - this PR only changes generated documentation styling.

Benchmark / Performance Impact

N/A - documentation-only change.

Notes for Reviewers

• The PR intentionally changes only the local CSS override file.
• No logo assets, custom Doxygen header, or vendored third-party theme files are added.
• The local preview service used for review was http://127.0.0.1:8787/.