Custom header or paragraph style?

Is there a way to create a custom header or paragraph style that I can apply globally to a word or sentence?

Sure. Here's an idea.

Let's say the sentence in question is, "I like turtles."  The code below will find all instances of this sentence in headers of any size and in paragraphs, regardless of capitalization, and color the text red while giving it a green background. Add it to your footer injection and any time you type this sentence, it should be red and green.

<style>
.turtles {
  color: red;
  background: green;
}
</style>
<script>
document.querySelectorAll("h1, h2, h3, h4, h5, h6, p").forEach(function(el){
    el.innerHTML = el.innerHTML.replace(/i like turtles/gi, function(text){
        return "<span class='turtles'>" + text + "</span>";
    })
})
</script>

Thank you for the response. 

It appears that would require me to upgrade my account to a business account or above.  

I was hoping to accomplish this without this cost. Oh well.