THE one thing that will help your website look more professional and less “home-made” is to have consistency of fonts, colors & spacing. If you don’t override it, your WordPress theme helps you with this. When a user calls up one of your pages, it tells the browser to display all the body (main area) content in a certain font, font color & font size. And it does the same with other elements of your site, including the spacing around those elements.
One way that you might inadvertently override your theme is when you cut & paste content into the content editor. WordPress has gotten a lot better at keeping “just the basics”, but sometimes you can still pull formatting that you didn’t intend to into a post or page. When that happens, just select that text, and click on the “Clear formatting” button on the bottom row of editor tools (don’t see a bottom row? then click on the “Toolbar Toggle” button at the right end of the top row). This should clear the formattting. If it doesn’t, cut that text out of the editor, paste it into a plain text editor like Notepad, copy it again (the formatting will have been removed), and paste back into WordPress.
Sometimes, website editors intentionally override the theme in an attempt to emphasize text. This almost always comes off in an unprofessional way. See this article on how to emphasize text “without seeming desperate or deranged”. Use all caps, bold and italics to make your point. These should get displayed in the font of your theme, and even if that theme gets changed should still emphasize in the same way. There is almost never a good reason to change the color of the font with the WP editor–say to red–which will certainly make your text stand out, but will not look good.