Improve styling

[UnnaturalTwilight]

Description

This makes more styles stack on others to make theming more consistent.

• 'current' and 'highlight' versions of symbols styles stack on the base style for that symbol
• Models now use text_color
• highlight_border_style defaults to borders_style
• tab bar defaults to using current_item_style for active tab and text_color for inactive tab

Only the model change should affect fully defined themes but the rest of the changes make theming more forgiving to incomplete themes and reduce the effects of adding new theme options as they inherit from the existing options.

Related issues

This is progress on #920 but not a full implementation of text color

Checklist

• All tests passed
• Code has been formatted with rustfmt (nightly)
• Code has been checked with clippy (stable)
• Documentation has been updated (if needed)
• Changelog has been updated

If these changes are ok I will work on a pr for the docs.