This is due to the inline CSS styles in your contact form, which is what controls the appearance of your form. Whoever created the contact form applied a large amount of inline styles. Use of inline styles is generally considered inefficient.
To change the style of your CF7 forms you would need to edit either these inline styles or preferably the CSS style sheets used by your WordPress theme.
See Styling Contact Form for a general explanation of styling CF7 forms using CSS. If you are not familiar with CSS, this page also includes some links to where you can learn CSS.
The most common and preferred method is to create a Child Theme and make any necessary additions to the CSS in the child themes style.css only, rather than directly in the themes styles.css. That way you don't loose your changes when you update the theme.
Use Firebug or Chrome Dev Tools to examine the HTML and CSS of your CF7 form in detail. Once you understand how the HTML and CSS is configured in your CF7 form, you should be able to see the CSS changes you need to change the appearance of the form to suit your requirements.
Also good idea to target CSS classes & ids used within your CF7 forms only, so that your CSS changes don't inadvertantly effect other elements on your website.
If you are uncomfortable editing CSS style sheets you may prefer to use a custom CSS plugin like My Custom CSS.