Changelog: added

Add preview for diffs colors

Move diff color preview to top

Refresh page if color scheme changed

Refine wording of diff color settings

Fix specs & lint

Refine wording of diff color settings (2)

Refactor diff color pickers and preview

Use custom suggested colors

Rename suggested colors

Extend diff preview with idiff content

Apply 2 suggestion(s) to 2 file(s)

Extract util function

Add preference specs

Use lock-retry methodology in DB migration

Polish UserPreference specs

Refactor diff colors

Polish PreferencesHelper specs

Revert removal of !important due to Dark theme

Add frontend specs

Fix RuboCop 'Layout/LineLength'

Adjust setting description

Polish frontend specs

Use Snapshot for diff preview test