fix: white text on white background in grid demo 3.0.0-RC.1

[nimrof]

🐛 Bug Report

When the demo site is viewed with dark theme is impossible to read the text inside the grid boxes

💻 Repro or Code Sample

View the examples in the grid demo page with dark theme
https://calm-sea-053fa6a03-365.westeurope.3.azurestaticapps.net/Grid

🤔 Expected Behavior

viewable text

😯 Current Behavior

💁 Possible Solution

Change background or font color

🔦 Context

🌍 Your Environment

• OS & Device: [e.g. MacOS, iOS, Windows, Linux] on [iPhone 7, PC]
• Browser [e.g. Microsoft Edge, Google Chrome, Apple Safari, Mozilla FireFox]
• .NET and FAST Version [e.g. 1.8.0]

[vnbaaij]

Fixed. See https://preview.blazor-fluentui.net/Grid (yes, that is a new url for the preview demo)