Ecosystem: switch default color scheme to Magenta 70 but allow changing

[Eric-Arellano]

Closes https://github.com/Qiskit/qiskit_sphinx_theme/issues/487.

It's important that we distinguish between core Qiskit and the Ecosystem. The new color scheme allows blue, pink, and purple. Projects more aligned with IBM, like qiskit-ibm-runtime, should use blue. Projects more separated from IBM should use more pink. And projects in-between could maybe stick with purple.

So, we use magenta for the default ecosystem color.

But, we define --qiskit-color-{purple,magenta,blue} so that themes can override the color. They can do this in conf.py using this code (the feature comes from Furo):

    html_theme_options = {
        "light_css_variables": {
            "color-brand-primary": "var(--qiskit-color-blue)",
        }
    }

All colors come from Carbon's color palette and use the shade 70 because it looks more professional ("utility") and coincides with using Purple 70 for Qiskit.