Add page background color option to jsPDF constructor and API

[h-rushi]

Summary

This enhancement should add support for setting a full-page background color in jsPDF, allowing users to create PDFs with colored backgrounds instead of the default white background.

Motivation

Currently, jsPDF only supports white page backgrounds. Many use cases require colored backgrounds for:

• Branded documents with company colors
• Dark mode PDF generation
• Design templates with specific color schemes
• Print materials requiring non-white backgrounds
• Accessibility improvements with high-contrast backgrounds

Implementation Details

Constructor Option

Users should be able to set a page background color when creating a new jsPDF instance:

// Using hex color
const doc = new jsPDF({
  pageBackgroundColor: '#ffcccc'
});

// Using RGB array
const doc = new jsPDF({
  pageBackgroundColor: [255, 204, 204]
});

// Using named colors
const doc = new jsPDF({
  pageBackgroundColor: 'lightblue'
});

New API Methods

Two new methods should be added to the jsPDF API:

// Set background color for future pages
doc.setPageBackgroundColor('#ccffcc');

// Get current background color setting
const currentColor = doc.getPageBackgroundColor();

[h-rushi]

This change is important because many libraries built on top of jsPDF only allow passing options and don’t expose full API access. This makes it hard to implement customizations like background color, which is a common need for branding, dark mode, or accessibility. Adding native support would simplify usage and improve flexibility for developers.