Attaching a Custom Theme Assembly
If you create a custom theme assembly using the ASPxThemeBuilder tool and wish to apply a theme from this custom assembly, you should reference the theme assembly within your project.
Right click your site's References folder in Solution Explorer and click Add Reference...
Click the Browse... button and select the required theme assembly.
This adds the assembly to the project's References folder. Remember to set this assembly's Copy Local property value to true in order to copy this assembly to the web server while deploying.
Assign the assembly name to the customThemeAssemblies option in the devExpress web.config section.
<devExpress> <themes customThemeAssemblies="MyThemeAssembly" ... /> ... </devExpress>
You can now apply the custom theme to your project.
A custom theme assembly should have the same minor/major version as other DevExpress assemblies used in a project. Otherwise, you should upgrade the theme assembly's source code, rebuild it, upgrade the corresponding references in the target MVC application. See the KA18887 KB Article for more information.