fix: use css variables for ingest page to support dark mode #49

[heytt-satra]

Description

This PR addresses the low color contrast issue on the "Ingest by Repository URL" page when viewed in dark mode, ensuring text is legible and meets accessibility standards.

Changes

• Replaced hardcoded hex color values in [frontend/src/pages/IngestPage.css] with application-wide CSS variables (e.g., var(--content-bg-color), var(--header-color), var(--input-bg)).
• The component now dynamically adapts to the active theme (Light/Dark) defined in [index.css]
• Added specific styles for disabled inputs to maintain readability in both modes.

Verification

• Visual Check: Verified that the Ingest form is clearly visible and has high contrast in the browser.
• Code Check: Confirmed all hardcoded hex values have been removed from IngestPage.css.

Fixes #49