I have a button and I want to style it when disabled using inline style only.
I know how to style using internal/external style.
Code below:
<input type="submit" value="Submit" disabled="disabled" />
Other than by explicitly changing the style with JavaScript (which violates your "inline CSS only" constraint): you can't.
The style attribute only contains the body of a rule-set. It can't do anything that you need a selector for.
<style></style>-block to add the CSS needed if you have control over the surrounding HTML.