Make blog and news title gradient selectable

[danielfdickinson]

When using custom colour schemes one might not want the default gradients for the news and blog titles, so make these each a param (site first, then page, then current default)

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[bowman2001]

Now that we can add a custom color scheme, it should also affect the gradients behind some titles.

To override or replace these title modifier classes with other hard-coded gradients, as you proposed, would be one way.

But because these changes clearly belong to the color scheme, I would like to add them there and change the CSS structure a little. The gradients will become CSS variables, which are also defined in the color scheme. This will take a little time.

[danielfdickinson]

Makes sense to me. Thank you for working on it.