To mark a section with a red box around it…
- Edit the page.
- Select HTML mode (top right of the edit box)
- Wrap the paragraph with divs as follows: <div class="urgent"> paragraph </div>
- Select Visual mode again.
- It won’t appear any different in the editor, but on the actual website it will work correctly.
That will put a red box around it. The style for that is defined in Appearance / Editor / Stylesheet, at the end of the file. Only change it if you’re reasonably comfortable with CSS.