[HtmlTargetElement(Attributes = "class")]
public class InlineClassTagHelper : TagHelper
{
    private readonly IFileProvider _fileProvider;

    public InlineClassTagHelper(IFileProvider fileProvider)
    {
        _fileProvider = fileProvider;
    }

    [HtmlAttributeNotBound, ViewContext]
    public ViewContext ViewContext { get; set; }

    public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
    {
        var classNames =
            output
                .Attributes["class"]
                ?.Value.ToString()
                .Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries)
                .Where(className => className.StartsWith("m-"))
                .Select(SoftString.Create)
                .ToImmutableHashSet();

        if (classNames is null)
        {
            return;
        }

        var cssFileName =
            $"wwwroot/css/" +
            $"{ViewContext.RouteData.Values["controller"]}." +
            $"{ViewContext.RouteData.Values["action"]}.css";

        var cssFile = _fileProvider.GetFileInfo(cssFileName);
        var cssString = await ReadFile(cssFile);
        var css = CssParser.Default.Parse(cssString);

        var declarations = css.Where(ruleset => ruleset.Selectors.Any(classNames.Contains)).Select(ruleset => ruleset.Declarations).Join(";");
        output.Attributes.SetAttribute("style", declarations);
        output.Attributes.RemoveAll("class");
    }

    private static async Task<string> ReadFile(IFileInfo file)
    {
        using (var reader = new StreamReader(file.CreateReadStream()))
        {
            return await reader.ReadToEndAsync();
        }
    }
}

<h2 class="m-title">@ViewData["Title"]</h2>

<h2 style="color: blueviolet;">About</h2>

m-title {
    color: blueviolet;
}