Allow overriding of default button style variables

[thegecko]

This PR adds explicit variables for the the colours of the default buttons.

This enables a user to override the default button colours, specifically the background colour using:

        :root {
            --button-color: var(--button-primary-color);
            --button-background: var(--accent-color);
            --border-color: var(--accent-color);
            --button-color-hover: var(--button-primary-color);
            --button-background-hover: var(--accent-color-hover);
            --border-color-richer: var(--accent-color-hover);
        }

They all default to the same as before, except for the hover border which is now set to #888888 to match skeleton.

This may be cleaner introducing a new hue for the default button, but I didn't want to break backwards compatibility nor have to deal with a 'transparent' hue colour.

[alex-r-bigelow]

Bump + context: I'm shopping for CSS frameworks, and this Skeleton fork seems the least dead + most relevant to what I'm looking for... but several totally sensible PRs like this hanging open for a year isn't a good sign.

@thegecko, are you still using Barebones, or have you moved on to something else?

[thegecko]

Not used if for a year :)