Consistent light and dark themes

## Expected Behavior

A light theme should have a dark font on a light background consistently

## Current Behavior

Currently many element of the UI are inconsistent, for example the document editor has a dark background. Switching betweem light and dark is very exhausting, especially for someone with astigmatism

## Possible Solution

Make the UI consistent, everything should be black on white or white on black and people should be able to switch between one and the other.

## Steps to Reproduce (for bugs)

1. Open http://localhost:5984/_utils/

## Context

I have astigmatism, the text becomes blurry when background is dark.

## Your Environment

* Version used: 3.1.1
* Browser Name and version: Chrome Version 87.0.4280.141 (Official Build) (64-bit)
* Operating System and version (desktop or mobile): Windows 10
* Link to your project:

Open http://localhost:5984/_utils/ in Chrome and compare the light and dark themes across the UI, especially the document editor. Identify the inconsistent foreground and background combinations, then verify that switching themes produces consistent readable contrast throughout the interface.

Written by the indexing model from the issue text.