New CSS Block - Following Tutorial

Hello - In following the tutorial delivered by Bootstrap Studio on YouTube ( Creating Apple’s Website in Bootstrap Studio 4 (Tutorial)), when attempting to create a new CSS Block, I noticed that the name doesn’t pull over with the already provided class name as it does in the video (.navbar.navbar-dark.navbar-expand-md.fixed-top.bg-dark), If I manually create the name, I receive a red error stating that the property is invalid as it has been overridden by the rule (.bg-dark). This is the same on the next step in the video. If you could point me in the right direction, I’d appreciate it.
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Welcome to the forum.

The video you are watching is over 3 years old, and there have been a number of updates to the software since it has been made. One of which is the css rule override notification. Also creating a style will now only give .navbar instead of the chain of classes.