`monobackgroundcolor` ignored by document title and section title

[zenggyu]

Bug description

As the title suggests, monobackgroundcolor seems to be ignored by by document title and section title.

Steps to reproduce

---
title: "Document title with `code`"
format:
  html:
    monobackgroundcolor: "transparent"
---

# Section title with `code`

Some text with `code`

Expected behavior

monobackgroundcolor should affect document title and section title as well.

Actual behavior

monobackgroundcolor does not affect document title or section title. Example below:

Your environment

• IDE: RStudio 2023.06.0 Build 421
• OS: Ubuntu 22.04

Quarto check output

quarto check

[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.1: OK
      Dart Sass version 1.55.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.3.361
      Path: /opt/quarto/bin

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.10.12
      Path: /usr/bin/python3
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
      Version: 4.3.0
      Path: /usr/lib/R
      LibPaths:
        - /home/nick/R/x86_64-pc-linux-gnu-library/4.3
        - /usr/local/lib/R/site-library
        - /usr/lib/R/site-library
        - /usr/lib/R/library
      knitr: 1.43
      rmarkdown: 2.22

[✓] Checking Knitr engine render......OK

[mcanouil]

I am not completely sure it is a bug, maybe an omission.

quarto-cli/src/resources/formats/html/bootstrap/_bootstrap-rules.scss

Lines 909 to 937 in f87976c

	@if variable-exists(mono-background-color) {
	p code:not(.sourceCode),
	li code:not(.sourceCode),
	kbd,
	pre:not(.sourceCode),
	samp {
	background-color: $mono-background-color;
	padding: 0.2em;
	}
	}
	// Default padding if background is set
	p code:not(.sourceCode),
	li code:not(.sourceCode),
	td code:not(.sourceCode) {
	@if variable-exists(mono-background-color) {
	background-color: $mono-background-color;
	} @else if variable-exists(code-bg) {
	background-color: $code-bg;
	}
	@if variable-exists(code-padding) {
	padding: $code-padding;
	} @else if variable-exists(code-bg) {
	padding: 0.2em;
	} @else if variable-exists(mono-background-color) {
	padding: 0.2em;
	}
	}

[mcanouil]

I made a PR to extend monobackgroundcolor to HTML headers.