If you have budget available, you may want to hire a developer to create a theme for you. If there’s no budget, browse through the WordPress.org theme repository to find a theme that looks similar to the rest of your website. Don’t worry too much about colors, think more about the overall layout – the placement of things and the size of the header. Some themes offer settings that will allow you to change the color scheme to more closely match your site. Others let you add custom CSS so you can tweak the colors that way.
If you find a theme that’s close but the functionality isn’t quite there, you may eventually want to consider creating a child theme – this takes most of the parent theme but lets you override whatever parts and pieces you wish. However this requires more knowledge of both PHP and CSS, so a theme that allows you to tweak colors and/or CSS is probably the easiest route.