Fixed in version Oxygen 3.2!
After the release of WordPress 5.3 in the Oxygen interface for any elements, the color picker began to have an oval shape (or the shape of an "egg", as this phenomenon was called in the Oxygen community). This is due to a change in styles in WordPress core files. It amuses someone, but it can annoy someone.
I suggest a simple trick to work around this problem temporarily. In WordPress, in the "Appearance> Customize> Additional CSS" section, add the following CSS code:
This action will return the normal view to the color picker in the editor interface. Once this bug is fixed in the next official Oxygen update, delete this code. But if you forget to delete it, then nothing bad will happen.
Do not try to add this CSS in the Code Block or to the StyleSheet, as this will not fix the problem in the editor interface.