Thank you for the provided screenshots.
After inspecting them I can assume that the "Material Black" theme is used. As the screenshots show different result then the expected ones, please noticed if the all of the style files are loaded and if the styles and the scripts are the same versions.
Also, inspect the Calendar elements to check if there are custom styles which are affecting the appearance:
If the issue still occurs, please provide an example demonstrating the issue, and I will gladly investigate what may be causing it.