[FEATURE REQUEST] Add configuration option for title color

[ryanlovett]

Is your feature request related to a problem? Please describe.
I'm exploring using this theme with your MyST template for a university course. We are able to swap out logos easily enough, but we'd also like to be able to customize the title text color as well.

Describe the solution you'd like
Perhaps a MyST template.yml option title_color as an RGB that gets propagated through to this package's beamercolorthemepureminimalistic.sty as pureminimalistic@text@title.

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