CSS or Cascading Style Sheets are great. CSS is a file which you can link to in the head of your document. Although styles can be applied inline within your HTML using the <style></style> tag, it is better form to link to an external style sheet. This can be done using the <link> tag eg. <link rel=”stylesheet” href=”style.css”>. By using an external style sheet, you can take advantage of the most powerful aspect of CSS which is that it can apply a uniform aspect to your entire site. If your page background is white and you want to change it to grey, you need only change a single line in you CSS file rather than have to go in and change every page individually. The other important aspect of CSS is that it allows you to separate content which should be in your HTML from presentation which is handled by your CSS. A nice free tutorial can be found at W3Schools.
Some useful CSS cleaners were recently described in a Sitepoint article 10 Terrific Time-Saving CSS Cleanup Tools by Peter North.