[AlexAndBear]

Description

Since the root got color-scheme: dark, Chromium paints an opaque white backdrop behind the book iframe, which the dark theme's invert filter doesn't reach. The book iframes now keep a light color scheme, so the backdrop stays transparent and the inverted text shows on the app surface again (this also removes the black box Firefox and WebKit showed).

Related Issue

• Fixes EPUB reader shows a blank white page in the dark theme in Chromium #3881

How Has This Been Tested?

• test environment: unit tests (Vitest), browser tests (Playwright) against a local OpenCloud
• test case 1: browser: dark theme in Chromium, Firefox and WebKit shows white text on the app surface (page turns, chapters, search, font size, slider, mobile); light theme unchanged

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)