Can we assign css to dynamic Generated Classes? YES we can

I have a class called .form-123 where the number changes every time the screen is refreshed. Is there a way to define this field (say background-color or margin) in the CSS?

Yes it is possible just by using CSS only, and there is 3 ways to do that

Option #1 - Match by prefix value

Use CSS Class selector ^="class" which select all elements whose class is prefixed by "form-"

example

[class^="form-"] {
  background: red;
  height: 100px;
  width: 100px;
  margin: 10px 0;
  display:block
}

<div class="box-123"></div>
<span class="box-124"></span>
<article class="box-125"></article>

Option #2 - Match by contains at least one value

Use another CSS class selector *="class" (equivalent to CSS attribute selector) which select all elements whose class contains at least one substring "form-".

[class*="form-"] {
  background: red;
  height: 100px;
  width: 100px;
  margin: 10px 0;
  display:block
}

<div class="form-123"></div>
<span class="form-124"></span>
<article class="form-125"></article>

Option #3 - Add an additional class to the element, then both those elements will have the class' CSS attributes:

.form-123 {
  background: blue;
  height: 100px;
  width: 100px;
  margin: 10px 0;
  display:block
}

extra-class {
  background: blue !important;
}