New themes for miA, and lots of polish

[playerofgames]

• Two brand new theme variants: 'mia Sans' and 'mia Serif', which use IBM Plex Sans and Serif respectively.
• Original 'mia' theme now called 'mia Quattro'.
• Tables are now styled to be consistent with each theme's aesthetic.
• Refined Heading styles and spacing.
• Document mode .
• Hierarchy section is now rendered using content font.
• The system font is used for popups (reference lookup, search, etc.).
• Numerous other changes to improve the experience when using the themes, including:
  • Sidebar scrollbar only becomes visible when mouse enters sidebar.
  • Popup menus are more visible.
  • Text in content and sidebar is darker in light theme mode.
  • Highlight colour lightened slightly.
  • Simplified admonition icon styling.
  • Journal heading spacing is consistent with regular page headings.