The _ViewImports.cshtml file contained within the root of each Plato theme let's theme developers import additional .NET namespaces or use within the theme layout files.

You can see the default _ViewImports.cshtml file contents below...

@using Plato.Internal.Abstractions.Settings
@using Microsoft.AspNetCore.Html
@inherits Plato.Internal.Layout.Razor.RazorPage<TModel>
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
@addTagHelper *, Plato.Internal.Layout