Escape CSS class names

CSS class names can't contain the `:` character. For example, it's not possible to declare the following class in CSS:

.lg:flex {
    ...
}

However we can use the `\` character to correct it:

.lg\:flex {
    ...
}

The class name is usable in HTML as usual:

<div class="lg:flex">    ...</div>

Using `\` to escape CSS class names has been used a lot in some CSS frameworks such as Tailwind.