Code Block Colors

Hello everyone,
I have created some custom CodeBlocks so I can change the size of the text in the block.
Now I really like how some block styles like ‘Shell’ ‘csharp’ etc have color themes to them.
I been looking through the code and i can see where they were able to set this up.
How can I add color to my custom code block i created? currently all the font is just white but it is sized correctly.
Here is an example of 1 that i made.

.app-container .markdown-preview-view pre.language-fs12, 
.app-container .markdown-preview-view pre.language-fs12 code.language-fs12, 
.app-container .markdown-source-view pre.language-fs12 code.language-fs12,
.fs12 {
   font-size: 12px;
}
.app-container .markdown-preview-view .fs12 {
   font-size: 12px;
}
.app-container .markdown-preview-view pre[class*="language-fs"] code[class*="language-fs"] {
  color: var(--text-normal);
  white-space: pre-wrap;
  font-family: var(--default-font);
  padding: 0;
  background:none;
}

I the use it like this `` `fs12``` (without the space)

I would like to have it be the same style as 1 of the current code blocks Shell or whatever one

Thank you