Migrate UserPreference settings with the "default" theme to "light"

This was missed when upgrading OpenProject at some point so this should
bring everything back in order for users that still have the "default"
theme setting which is no longer supported, and keep the preferences
for said users who have since moved to one of the valid optionsl like
"light" or "light_high_contrast".

db/migrate/20240909151818_migrate_legacy_default_theme_to_light_theme.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+class MigrateLegacyDefaultThemeToLightTheme < ActiveRecord::Migration[7.1]
+  class MigrationUserPreference < ActiveRecord::Base
+    self.table_name = "user_preferences"
+  end
+
+  # Migrate legacy `default` theme to `light` theme
+  #
+  # Moving from:
+  # {
+  #   "settings": {
+  #     "theme": "default"
+  #   }
+  # }
+  #
+  # To:
+  # {
+  #   "settings": {
+  #     "theme": "light" # current default theme
+  #   }
+  # }
+  def up
+    MigrationUserPreference
+      .where(settings: { theme: "default" })
+      .update_all(settings: { theme: "light" })
+  end
+
+  # no-op
+  def down; end
+end

spec/migrations/migrate_legacy_default_theme_to_light_theme_spec.rb

@@ -0,0 +1,56 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+require Rails.root.join("db/migrate/20240909151818_migrate_legacy_default_theme_to_light_theme.rb")
+
+RSpec.describe MigrateLegacyDefaultThemeToLightTheme, type: :model do
+  RSpec::Matchers.define_negated_matcher :not_change, :change
+
+  # Silencing migration logs, since we are not interested in that during testing
+  subject(:run_migration) { ActiveRecord::Migration.suppress_messages { described_class.new.up } }
+
+  shared_let(:user_with_default_theme) { create(:user, preferences: { settings: { theme: "default" } }) }
+  shared_let(:user_with_light_theme) { create(:user, preferences: { settings: { theme: "light" } }) }
+  shared_let(:user_with_light_high_contrast_theme) { create(:user, preferences: { settings: { theme: "light_high_contrast" } }) }
+  shared_let(:user_with_dark_theme) { create(:user, preferences: { settings: { theme: "dark" } }) }
+
+  it "sets the theme to light for users with the 'default' theme" do
+    expect { run_migration }
+      .to change { user_with_default_theme.reload.pref.settings["theme"] }
+      .from("default")
+      .to("light")
+  end
+
+  it "does not change the theme for users with another theme that isn't 'default'" do
+    expect { run_migration }
+      .to not_change { user_with_light_theme.reload.pref.settings["theme"] }
+      .and(not_change { user_with_light_high_contrast_theme.reload.pref.settings["theme"] })
+      .and(not_change { user_with_dark_theme.reload.pref.settings["theme"] })
+  end
+end