Add ResourceDictionary to class library

create a resource dictionary like this one

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">  <ResourceDictionary>    <ResourceDictionary.MergedDictionaries>      <!-- Common base theme -->      <ResourceDictionary Source="pack://application:,,,/Another.AssemblyName;component/YourResDictionaryFolder/OtherStyles.xaml" />      <ResourceDictionary Source="pack://application:,,,/Another.AssemblyName;component/YourResDictionaryFolder/AnotherStyles.xaml" />    </ResourceDictionary.MergedDictionaries>  </ResourceDictionary>  <!-- store here your styles --></ResourceDictionary>

and you can put it where you want

<Window x:Class="DragMoveForms.Window2"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        Title="Window2"        Height="300"        Width="300">  <Window.Resources>    <ResourceDictionary Source="pack://application:,,,/Your.Base.AssemblyName;component/YourResDictionaryFolder/Dictionary1.xaml" />  </Window.Resources>  <Grid>  </Grid></Window>

To transform a classical library project to a WPF library project (in order to add UserControls, Windows, ResourcesDictionaries, etc.) you can add the following XML in the .csproj file in the first PropertyGroup Node :

<ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>

Full example :

<?xml version="1.0" encoding="utf-8"?><Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />    <PropertyGroup>      <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>      <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>      <ProjectGuid>{50E8AAEA-5CED-46BE-AC9A-B7EEF9F5D4C9}</ProjectGuid>      <OutputType>Library</OutputType>      <AppDesignerFolder>Properties</AppDesignerFolder>      <RootNamespace>WpfApplication2</RootNamespace>      <AssemblyName>WpfApplication2</AssemblyName>      <TargetFrameworkVersion>v4.6.1</TargetFrameworkVersion>      <FileAlignment>512</FileAlignment>      <ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>      <WarningLevel>4</WarningLevel>      <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>      <TargetFrameworkProfile />    </PropertyGroup>    <!-- ... -->    

@punker76's answer is great and helped me a lot, but it's worth adding that if you create an empty file and add a resource tag into it you should also go to file properties, set BuildAction to Resource, Copy To... to Do not copy and clear CustomTool property if it's set.