Feature: color mode support for shiki

## Description

Currently, I identify two issues with Shiki Code Hightlight:
1. I am unable to change the colors on the go (dark/light mode toggle).
- I tried multiple approaches. Maybe I'm doing something wrong.
1. The colors are stored in style attribute as rgb values. This means they will remain in the scheme they are written in even after exporting to html.
- I export html and store it in the db, then display html instead of rendering lexical editor in non-edit mode.

Users will be able to see the code highlight in their preferred mode (potentially even theme).

Use `vars` instead of rgb values.

## Impact

Basically anyone with dual color scheme.

No file or test is named in the issue. Start by locating the Shiki code-highlighting integration and trace how it produces inline RGB styles; then review the exported HTML path and color-mode behavior. Done means highlighting responds to dark/light mode changes and exported HTML preserves a mode-appropriate, changeable color scheme.

Written by the indexing model from the issue text.