You have selected a theme for your blog. You are excited to show it to the world. But I would suggest you to hold it for a little more and make sure the theme is ready before you make it live. So if you are planning to change the theme for your blog, I would suggest you to go through the following checklist before you make it live.
Create a Child Theme
Making changes to your core theme files may be simple but not recommended. Because when you need to upgrade your theme, you will have to apply those changes again making the whole upgrade process cumbersome. This is where Child Theme steps in. Child Theme inherits everything from the Parent theme and if you want to make any change in the original theme, you can do it from the child theme without touching the actual theme. Here’s a more on Child Themes you can read from.
WordPress 3.4 shipped with a nifty little feature called the Theme Customizer. You can use it to test your theme before making it live. It basically shows how your theme will look if you had activated it. You can even make some of the changes on the fly like Implementing menus, changing header and background, site title description and front page settings. Some themes come up with their own customization options like color and layout.
Theme Test Drive Plugin
If you want to do exhaustive testing which might take a long time, then the inbuilt Theme Customizer may prove ineffective. Because for that you will have to fire up the Customizer everytime and browse your site inside that which can be cumbersome. Theme Test Drive plugin allows you to browse your site with a theme of your own choice which only you will see while others will see the old theme. This way you can browse your site and keep making changes without breaking anything for your audience.
Check all Pages
When I say all pages it doesn’t mean every single page on your site. You need to check for single post, front page, search page, 404 pages, sitemap/contact pages, date/day/month archives, category/taxonomy archives, custom post type pages/archives. Sometimes people just check frontpage and single post and think that if these are working, means site is working as well. You need to make sure all pages are loading properly and no bugs are encountered on these. Also check your archive pages for excerpts/content to see if they show up how you like them. Also check for title tags because title tags implementation is different in themes and you might end up with wrong or improper tags especially when they mess up with the settings of the SEO plugin you are using.
You need to check for any plugins that might mess up your theme. Plugins like contact forms, archive/sitemap or any widget based plugins are the most common ones that need to be looked at when you install a new theme. You may need to make appropriate css changes to your theme to adjust your site’s look according to the plugin you use. Some themes also support some plugins which you might already be using. You need to check whether that is working properly or not.
Widgets and Menus
Whenever you change your theme, make sure the widgets and the menus are properly updated and are working with your new theme as well. Some widgets might need to be changed/updated or even redesigned according to the new theme. This is even more important if your widgets contain social media information or ads whose format needs to be adjusted with the new theme.
Customizations from Old Theme
Sometimes you have customizations made in your old theme which you might want to carry forward. Mostly such customizations are made in functions.php. Now this is a bad practice of using your theme’s function.php for customizing it if those changes should work irrespective of the theme you have. It would make sense to have a plugin for such customizations. Anyways if you have changes from old theme which you want to carry forward, make a note of them and apply theme in your new child theme’s function.php or create a new plugin if those changes are theme independent.
You should ensure your site is still loading as fast as it was before you changed your theme. This is one test you can only perform after making your theme live. I would suggest you to use these tools to test your site’s speed: PingDom Tools and Web Page Analyzer