I have the problem which when I create a header in my wordpress plugin metabox, it always apply the default style on my header:
even when I apply, my own style, <h3 style="background-color: black; color:blue">Text Block Settings</h3>, it only changes the text color but not the background color, why??
This seems like a CSS Specificity issue although without access to the source code I can’t tell what’s going on.
Since it seems like you want a very hack-ey fix, try
<h3 style="background:black !important; color:blue">Text Block Settings</h3>and see if that works (normally you should avoid using!importantbut if this is a one-off override it can be alright).