"my technical skill is not that good": you're going to need to some knowledge of HTML and CSS. In short, you'll need to inspect the retry button to find a CSS class (or combination classes) to which you can apply new CSS style rules. You'll apply that new styling in the "Custom CSS/Less code" found in the project settings.
- Assuming that "the assessment retry button" is retry button from adapt-contrib-assessmentResults, the CSS class that you'll target is likely to be something like "results-retry-button".
- Inspect the Preview produced by the AT (or the running course). Use something like Dev Tools (press F12 key or Ctrl+Shift+I) to inspect the browser code.
- In the AT's Custom CSS/Less code, you'll add your new styles somewhat like this example:
border: 2px solid #000;
Hope this gets you a step further down your path.