diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index d8b69ee..5cc0624 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -8,12 +8,11 @@ on:
   pull_request:
 
 jobs:
-  build:
+  test:
     runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v4
-
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "3.3"
@@ -22,8 +21,32 @@ jobs:
       - name: Lint
         run: bundle exec rake standard
 
+  integration_test:
+    runs-on: ubuntu-latest
+
+    needs: test
+
+    strategy:
+      matrix:
+        include:
+          - ruby_version: "3.3"
+            redmine_branch_name: "master"
+
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: "3.3"
+          bundler-cache: true
+
       - name: Set up test
         run: bundle exec rake rexer:test:build_integration_test_image
+        env:
+          RUBY_VERSION: ${{ matrix.ruby_version }}
+          REDMINE_BRANCH_NAME: ${{ matrix.redmine_branch_name }}
 
-      - name: Run tests
-        run: bundle exec rake test
+      - name: Run integration tests
+        run: bundle exec rake test:integration
+        env:
+          RUBY_VERSION: ${{ matrix.ruby_version }}
+          REDMINE_BRANCH_NAME: ${{ matrix.redmine_branch_name }}
diff --git a/README.md b/README.md
index e01dd65..cf7f952 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,10 @@ It is mainly aimed at helping with the development of Redmine and its plugins, a
 gem install rexer
 ```
 
+## Supported Redmine
+
+Rexer is tested with Redmine v5.1 and trunk.
+
 ## Usage
 
 ### Quick Start
@@ -120,20 +124,26 @@ end
 
 ## Developing
 
-### Running integration tests
+### Running tests
 
 First, you need to build the docker image for the integration tests.
 
 ```
-rake rexer:test:build_integration_test_image
+rake test:prepare_integration
 ```
 
-Then, you can run the integration tests.
+Then, you can run all tests.
 
 ```
 rake test
 ```
 
+Or, you can only the integration tests as follows.
+
+```
+rake test:integration
+```
+
 ### Formatting and Linting code
 
 This project uses [Standard](https://github.com/standardrb/standard) for code formatting and linting. You can format and check the code by running the following commands.
diff --git a/Rakefile b/Rakefile
index 5336e15..0c0add6 100644
--- a/Rakefile
+++ b/Rakefile
@@ -2,20 +2,31 @@ require "bundler/gem_tasks"
 require "rake/testtask"
 require "standard/rake"
 
-Rake::TestTask.new(:test) do |t|
+Rake::TestTask.new("test:integration") do |t|
   t.libs << "test"
   t.libs << "lib"
-  t.test_files = FileList["test/**/*_test.rb"]
+  t.test_files = FileList["test/integration/**/*_test.rb"]
   t.warning = false
 end
 
 task default: %i[test standard]
 
-namespace :rexer do
-  namespace :test do
-    desc "Build the integration test image"
-    task :build_integration_test_image do
-      system "docker build -f test/integration/Dockerfile -t rexer-test .", exception: true
-    end
+desc "Run all tests"
+task test: %i[test:integration]
+
+namespace :test do
+  desc "Prepare to run integration tests"
+  task :prepare_integration do
+    ruby_version = ENV["RUBY_VERSION"] || "3.3"
+    redmine_branch_name = ENV["REDMINE_BRANCH_NAME"] || "master"
+
+    image_tag = "rexer-test:#{ruby_version}-#{redmine_branch_name}"
+
+    system(<<~CMD, exception: true)
+      docker build -f test/integration/Dockerfile \
+        --build-arg RUBY_VERSION=#{ruby_version} \
+        --build-arg REDMINE_BRANCH_NAME=#{redmine_branch_name} \
+        -t #{image_tag} .
+    CMD
   end
 end
diff --git a/lib/rexer/extension/theme.rb b/lib/rexer/extension/theme.rb
index 2d3ee40..44dd82d 100644
--- a/lib/rexer/extension/theme.rb
+++ b/lib/rexer/extension/theme.rb
@@ -2,7 +2,14 @@ module Rexer
   module Extension
     module Theme
       def self.dir
-        Pathname.new("themes")
+        public_themes = Pathname.pwd.join("public", "themes")
+
+        if public_themes.exist?
+          # When Redmine version is v5.1 or older, public/themes is used.
+          public_themes
+        else
+          Pathname.new("themes")
+        end
       end
 
       class Base
diff --git a/test/integration/Dockerfile b/test/integration/Dockerfile
index 5afd142..a926827 100644
--- a/test/integration/Dockerfile
+++ b/test/integration/Dockerfile
@@ -2,6 +2,8 @@ ARG RUBY_VERSION=3.3
 
 FROM ruby:$RUBY_VERSION
 
+ARG REDMINE_BRANCH_NAME=master
+
 RUN set -eux; \
     apt-get update; \
     apt-get install -y --no-install-recommends \
@@ -52,7 +54,7 @@ RUN git init && \
 # Redmine
 #
 WORKDIR /redmine
-RUN git clone --depth 1 https://github.com/redmine/redmine.git .
+RUN git clone -b $REDMINE_BRANCH_NAME --depth 1 https://github.com/redmine/redmine.git .
 COPY test/integration/docker/database.yml config/database.yml
 
 # Directory for the bundle cache
diff --git a/test/integration/docker/plugin_a/app/models/hello.rb b/test/integration/docker/plugin_a/app/models/hello.rb
index d766d5e..b4e9185 100644
--- a/test/integration/docker/plugin_a/app/models/hello.rb
+++ b/test/integration/docker/plugin_a/app/models/hello.rb
@@ -1,2 +1,2 @@
-class Hello < ApplicationRecord
+class Hello < (defined?(ApplicationRecord) ? ApplicationRecord : ActiveRecord::Base)
 end
diff --git a/test/integration/docker/plugin_a/db/migrate/20240813045236_create_hellos.rb b/test/integration/docker/plugin_a/db/migrate/20240813045236_create_hellos.rb
index 05e80a8..2e436ed 100644
--- a/test/integration/docker/plugin_a/db/migrate/20240813045236_create_hellos.rb
+++ b/test/integration/docker/plugin_a/db/migrate/20240813045236_create_hellos.rb
@@ -1,4 +1,4 @@
-class CreateHellos < ActiveRecord::Migration[7.1]
+class CreateHellos < ActiveRecord::Migration[6.1]
   def change
     create_table :hellos do |t|
       t.string :world
diff --git a/test/integration/integration_helper.rb b/test/integration/integration_helper.rb
index 25cd504..a41f3e0 100644
--- a/test/integration/integration_helper.rb
+++ b/test/integration/integration_helper.rb
@@ -3,6 +3,8 @@
 require "rake"
 
 ENV["VERBOSE"] = "1"
+ENV["RUBY_VERSION"] ||= "3.3"
+ENV["REDMINE_BRANCH_NAME"] ||= "master"
 
 module IntegrationHelper
   Result = Data.define(:output_raw, :error_raw, :status_raw) do
@@ -15,7 +17,9 @@ def error = error_raw.strip
     def success? = status_raw.success?
   end
 
-  def image_name = "rexer-test"
+  def image_name
+    @image_name ||= "rexer-test:#{ENV["RUBY_VERSION"]}-#{ENV["REDMINE_BRANCH_NAME"]}"
+  end
 
   def container_name = "rexer-test-container"
 
@@ -62,4 +66,13 @@ def docker_exec(*command, raise: false)
   def docker_stop
     run_with_capture("docker container stop -t 0 #{container_name} && docker container rm #{container_name}", raise: true)
   end
+
+  def legacy_theme_dir?
+    return @legacy_theme_dir if defined?(@legacy_theme_dir)
+    @legacy_theme_dir = docker_exec("test -d /redmine/public/themes").success?
+  end
+
+  def theme_dir
+    legacy_theme_dir? ? "/redmine/public/themes" : "/redmine/themes"
+  end
 end
diff --git a/test/integration/integration_test.rb b/test/integration/integration_test.rb
index 7d12a79..8d1ae9d 100644
--- a/test/integration/integration_test.rb
+++ b/test/integration/integration_test.rb
@@ -43,8 +43,8 @@ class IntegrationTest < Test::Unit::TestCase
       assert_equal "plugin_a", result.output.last
     end
 
-    docker_exec("ls themes").then do |result|
-      assert_equal "theme_a", result.output.last
+    docker_exec("ls #{theme_dir}").then do |result|
+      assert_includes result.output.last, "theme_a"
     end
 
     docker_exec("bin/rails r 'puts Hello.table_name'").then do |result|
@@ -60,8 +60,9 @@ class IntegrationTest < Test::Unit::TestCase
       assert_equal ["README"], result.output
     end
 
-    docker_exec("ls themes").then do |result|
-      assert_equal ["README"], result.output
+    docker_exec("ls #{theme_dir}").then do |result|
+      expected_files = legacy_theme_dir? ? %w[README alternate classic] : %w[README]
+      assert_equal expected_files, result.output
     end
 
     docker_exec("rex state").then do |result|
@@ -120,7 +121,7 @@ class IntegrationTest < Test::Unit::TestCase
       assert_equal "update", result.output_str
     end
 
-    docker_exec("cat /redmine/themes/theme_a/README").then do |result|
+    docker_exec("cat #{theme_dir}/theme_a/README").then do |result|
       assert_equal "update", result.output_str
     end