use background property and custom vars for code block

[rammodhvadia]

uses the following vars for custom styling:
--code-block-blend-mode: set custom blend mode for highlighted lines of code in code blocks, defaults to difference
--code-block-line-highlight-border: allows setting border value for border-inline-start on code blocks. Used to add a bar to the start of a highlighted line, defaults to none
--code-block-line-highlight-border-color: sets the color of the border, defaults to none

Also uses the background property instead of background-color to remove colour gradient from code block highlight

Example of this in use in cc projects

[Copilot]

Pull request overview

This PR updates the .line-highlight styling in Instructions.scss to use CSS custom properties for greater customization of code block line highlights. It also changes background-color to the background shorthand to remove color gradients, and applies minor formatting cleanups (hex color casing, spacing).

Changes:

• Replaces background-color with background shorthand and makes mix-blend-mode configurable via --code-block-blend-mode custom property
• Adds two new CSS custom properties (--code-block-line-highlight-border and --code-block-line-highlight-border-color) for customizing the inline-start border on highlighted lines
• Minor formatting: lowercases hex colors and adds a space in a selector

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[Copilot]

border-color: none is not valid CSS — the border-color property does not accept none as a value. When the browser encounters this, it will silently ignore the declaration. The fallback should be transparent instead of none.

Additionally, border-color is a physical shorthand that sets all four sides' colors (border-top-color, border-right-color, etc.), but it does not reliably override the color component of the logical border-inline-start property above it. To correctly override the inline-start border color, use border-inline-start-color instead of border-color.

Suggested change

	border-color: var(--code-block-line-highlight-border-color, none);
	border-inline-start-color: var(--code-block-line-highlight-border-color, transparent);