Question:
WordPress is up and running on my host — but it completely mismatches my site’s existing theme/CSS. How hard would it be to modify/write a new theme to make it fit in? Is there an easier way to do this?
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It might take some time to wrap your head around the theming system but it boils down to defining a stylesheet file for the theme and at least one PHP page template based on the master ‘look’ of your site. Add PHP code in strategic places to insert the title, posts, widgets, etc. and let WordPress do the heavy lifting. Look at other themes as examples, particularly the ones that claim to be simple and CSS-oriented, they should be easier to decipher.
Everything you need to know is here:
http://codex.wordpress.org/Theme_Development
You can do it without doing any coding. Get Artisteer theme builder and make your own themes the easy way. You can use your existing graphics and colors in Artisteer to replicate your current site. But I have to warn you, using Artisteer is very addictive! They have a free download so you can try a fully functional copy before you buy it. It will be one of the best investments you’ll ever make.
You can use a very good tutorial “How To Create a WordPress Theme” at http://themeshaper.com/wordpress-themes-templates-tutorial/.
Hope it helps,
Huy Do