Hi
The template declares this style to have that first letter:
.item-page .com-content-article__body > p:first-child::first-letter, .item-page .com-content-article__body p.has-letter::first-letter {
color: var(--color-primary);
float: left;
font-family: var(--heading-font-family);
font-size: 96px;
font-weight: 400;
line-height: 0.7;
margin-right: 16px;
padding-top: 16px;
}
You can override the font-size and color as you wish.
and put your custom CSS into the file: /templates/ja_blockk/css/custom.css (Create this file if it doesn't exist now)