This PR adds a new plot_theme configuration option that allows users to customize the visual appearance of all plots in MultiQC reports using any registered Plotly theme.

• Added plot_theme configuration option (accepts any Plotly theme name like "plotly_dark", "ggplot2", "seaborn", etc.)
• Created a custom MultiQC default plotly theme that preserves existing styling when no theme is specified
• Refactored plot initialization to use templates instead of hardcoded layout properties
• Updated configuration schema and documentation

Users can now set their preferred theme, eg: plot_theme: "plotly_dark"

When no theme is specified, plots use the same styling as before. When a theme is set, all plots consistently use that theme's styling.

Fixes #3162