fix: change all theme references to use --oscd-theme-*

[stee-re]

Shell was making hard references to colour variables which were not --oscd theme colours, thus preventing distros from being able to customize them. This PR aims to correct that.

Whats changed:

• Shell (and relatives) reference only oscd-* variables
• Shell now sets all the --oscd-* variables based on their --oscd-theme_* counterparts, falling back to sensible defaults.
• Shell sets all the --md-* and --mdc-* defaults based on --oscd-* variables.
• Setting of theme variables is done within the scope of the demo (for reference)

[ca-d]

LGTM, thanks!