Add instructions for using the dark theme selector

[jensjvh]

Description

What?

Add a section to the course page styling chapter for adding dark theme specific styles.

Implements #80

Testing

Remember to add or update unit tests for new features and changes.

• How to test your changes in A-plus
• How to test accessibility

What type of test did you run?

• Accessibility test using the WAVE extension.
• Django unit tests.
• Playwright tests.
• Other test. (Add a description below)
• [ x] Manual testing.

[ADD A DESCRIPTION ABOUT WHAT YOU TESTED MANUALLY]

Did you test the changes in

• Chrome
• Firefox
• This pull request cannot be tested in the browser.

Think of what is affected by these changes and could become broken

Translation

• Did you modify or add new strings in the user interface? (Read about how to create translation)

Programming style

• Did you follow our style guides?
• Did you use Python type hinting in all functions that you added or edited? (type hints for function parameters and return values)

Have you updated the README or other relevant documentation?

• documents inside the doc directory.
• README.md.
• Aplus Manual.
• Other documentation (mention below which documentation).

Is it Done?

• Reviewer has finished the code review
• After the review, the developer has made changes accordingly
• Customer/Teacher has accepted the implementation of the feature

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[ihalaij1]

Nice!

We should also add some examples of using Bootstrap 5 CSS variables, so that users can make use of the same values that A+ already uses in some places for dark and light mode:

.my-selector {
    color: var(--bs-body-color);
    background-color: var(--bs-body-bg);
    border-radius: var(--bs-border-radius);
}