So I noticed that the Telerik Loader isn't displaying the correct colour when you set the ThemeColor on it to the secondary colour, seemingly in any of the themes.
Here's a REPL I made, where each theme color is used, and I placed a button right next to each loader for easy comparison: https://blazorrepl.telerik.com/mGbYbMYa13VfwDlm06
Try changing the theme and running the repl again and you will notice it is consistently displaying as a very different colour to the button.
In this screenshot, the loader for Secondary is dark, whereas the button is much lighter. This appears to be an issue in all the themes I selected, but this screenshot shows the default theme. Notice, that all other loaders and buttons match at all times in all themes.