public DiagramTheme Theme { get; set; }

Public Property Theme As DiagramTheme

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
        xmlns:dxdiag="http://schemas.devexpress.com/winfx/2008/xaml/diagram" 
        dx:ThemeManager.ThemeName="MetropolisLight"
        x:Class="CustomShapeThemes.MainWindow"
        Title="MainWindow" Height="700" Width="1100">
    <Grid>

        <dxdiag:DiagramDesignerControl SelectedStencils="BasicShapes, BasicFlowchartShapes" Name="diagramControl"/>

    </Grid>
</Window>

using DevExpress.Diagram.Core;

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace CustomShapeThemes
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            InitCustomShapeTheme();
        }

        void InitCustomShapeTheme()
        {
            ResourceDictionary customShapeThemeDictionary = new ResourceDictionary() { Source = new Uri("CustomShapeTheme.xaml", UriKind.Relative) };
            ThemeRegistrator.RegisterThemes(customShapeThemeDictionary, (name) => name);
            diagramControl.Theme = ThemeRegistrator.GetTheme("MyTheme");
        }
    }
}

<p:ResourceDictionary xmlns="http://schemas.devexpress.com/winfx/2008/xaml/diagram"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:p="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
    <Theme x:Key="{ThemeKey MyTheme}" FontSize="10" FontFamily="Calibri">
        <ColorPalette Dark="Green" Light="LightGray" Accent1="Red" Accent2="Blue" Accent3="Brown" Accent4="Orange" Accent5="Violet" Accent6="DarkCyan" />
        <Theme.Effects>
            <EffectCollection>
                <EffectCollection.ThemeEffects>
                    <Effect Background="Light" Foreground="Accent" Stroke="ChangeBrightness(Accent, 0.5)" />
                    <Effect Background="Light" Foreground="Accent" Stroke="ChangeBrightness(Accent, 0.25)" />
                    <Effect Background="Light" Foreground="Accent" Stroke="Accent" />
                    <Effect Background="Accent" Foreground="Light" Stroke="Color('#FFC8C8C8')" />
                    <Effect Background="ChangeBrightness(Accent, -0.13)" Foreground="Light" Stroke="ChangeBrightness(Accent, -0.25)" />
                    <Effect Background="ChangeBrightness(Accent, -0.13)" Foreground="Light" Stroke="ChangeBrightness(Accent, -0.25)" />
                </EffectCollection.ThemeEffects>
                <EffectCollection.VariantEffects>
                    <Effect Background="Light" Foreground="Accent4" Stroke="Color('#FFC8C8C8')" />
                    <Effect Background="Accent1" Foreground="Accent6" Stroke="Accent1" />
                    <Effect Background="ChangeBrightness(Accent2, -0.13)" Foreground="Light" Stroke="ChangeBrightness(Accent1, -0.2)" />
                    <Effect Background="Accent3" Foreground="Light" Stroke="Color('#FFC8C8C8')" />
                </EffectCollection.VariantEffects>
                <EffectCollection.ConnectorEffects>
                    <Effect Background="Light" Foreground="Accent" Stroke="Accent" />
                    <Effect Background="Light" Foreground="Accent" Stroke="Accent" StrokeDashArray="4,4" />
                    <Effect Background="Light" Foreground="Accent" Stroke="Accent" StrokeThickness="2" />
                </EffectCollection.ConnectorEffects>
            </EffectCollection>
        </Theme.Effects>
    </Theme>
</p:ResourceDictionary>

Imports DevExpress.Diagram.Core

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports System.Threading.Tasks
Imports System.Windows
Imports System.Windows.Controls
Imports System.Windows.Data
Imports System.Windows.Documents
Imports System.Windows.Input
Imports System.Windows.Media
Imports System.Windows.Media.Imaging
Imports System.Windows.Navigation
Imports System.Windows.Shapes

Namespace CustomShapeThemes
    ''' <summary>
    ''' Interaction logic for MainWindow.xaml
    ''' </summary>
    Partial Public Class MainWindow
        Inherits Window

        Public Sub New()
            InitializeComponent()
            InitCustomShapeTheme()
        End Sub

        Private Sub InitCustomShapeTheme()
            Dim customShapeThemeDictionary As New ResourceDictionary() With {.Source = New Uri("CustomShapeTheme.xaml", UriKind.Relative)}
            ThemeRegistrator.RegisterThemes(customShapeThemeDictionary, Function(name) name)
            diagramControl.Theme = ThemeRegistrator.GetTheme("MyTheme")
        End Sub
    End Class
End Namespace

SelectedStencils="{Binding Path=(dxdo:DockLayoutManager.DockLayoutManager).DataContext.SelectedDiagramStencils, RelativeSource={RelativeSource Self}, Mode=TwoWay, Delay=400}"
Theme="{Binding Path=(dxdo:DockLayoutManager.DockLayoutManager).DataContext.DiagramTheme, RelativeSource={RelativeSource Self}, Mode=TwoWay}">
<dxmvvm:Interaction.Behaviors>