Apple - remove highlighting from Stickies?

I suggest that you try the “Paste and match style” menu command, right below “Paste” - the corresponding shortcut is ⇧–⌥–⌘-V. It will paste the content, but strip all the associated formatting.

You make a new sticky note, type in a word (for example "test") and select it.
Click right, go to fonts and then style.
Make the word in the style you want and add it to your favourite styles and give it a name (for example "MyStyle"). And save it.
Now, delete the sticky your just made.

Go to your sticky with the "problem" and select all.
Right-click and go to fonts and style
Click on favorite styles and select "MyStyle"
Click apply

This should do the trick.

Highlight all the text and right-click, go to font>styles and you can just click 'apply' to make all the styles match.