Is there anybody out there who can assist with Dreamweaver. It does not apply any changes at all.

It goes without saying that you should NOT be editing native Bootstrap CSS or JS files. Doing so could cause total failure of the framework.

If you must make changes to Bootstrap's default styles, use the pre-defined Bootstrap Helper Classes. See below.

https://www.w3schools.com/bootstrap/bootstrap_ref_all_classes.asp

Or in rare cases, use a separate stylesheet for your custom CSS code. 

<!--Minified Bootstrap CSS-->
<link href="css/bootstrap.min.css" rel="stylesheet">

<!--CUSTOM STYLES-->
<link href="css/custom_styles.css" rel="stylesheet">

Validate document and fix reported errors.

Go to Window > Results > Validation...  Check document.

Are you saving files after making changes (Cmd + S)?

Are you refreshing your screen with F5 (function keys enabled)?

...it just does not apply and becomes totally unresponsive.

By @joank32284658

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Ensure that your site is defined (Site > Manage Sites).  And the local site folder is read & writable on your Mac HD.

If you continue to have problems, Restore Preferences.

https://helpx.adobe.com/dreamweaver/kb/restore-preferences-dreamweaver.html

I had no less than 4 adobe agents looking into the matter, they have no clue.

By @joank32284658

Sounds about right!

Quite often, when style rules are not applied to your document, the reason is that the external file has bot been linked to the document.

As an example, if the external file is called styles.css, then there should be a link in the head section of your document that reads similar to <link rel="stylesheet" href="/css/styles.css">.

How are you changing the files? Are you saying it is freezing editing a CSS file? Inline styles? Are the files locally stored on your computer or are they stored on a network or online storage somewhere? What Mac OS are you using?