The so-called customizer is found in Appearance -> Themes. Many users complain that the tool won’t load. This is a bug that WP so far hasn’t fixed, but (unless you have the Custom Design upgrade and you want to use the color/font tools) it’s insignificant: you don’t really need the tool. [*]
The customizer simply brings together some options that are already available via the main dashboard sidebar: [*]
• You edit the blog title & tagline in Settings -> General.
• You set a static front page (or revert to latest posts on the homepage) in Settings -> Reading.
• You set a header image, and hide the blog title & tagline or adjust their color (if the theme supports these options), in Appearance -> Header.
• You set a background color or a background image in Appearance -> Background.
• You create and activate a custom top menu in Appearance -> Menus.
Note: the above options work in all themes, or in the majority of them. Some themes have additional options of their own (alternative color schemes or alternative layouts or other unique features). These are found in Appearance -> Theme Options. [*]
The customizer only brings together some basic options. [*] There are several other options you can or should take care of, especially when starting a new blog. Among other things:
• You make your name link to your blog, and set the interface language, in Users -> Personal Settings.
• You set the blog language, set your timezone, change the time and date format, and set a blavatar (= favicon, blog pic), in Settings -> General.
• You edit your displayed name, add profile info, and set a gravatar (= avatar, user pic), in Users -> My Profile.
• You set the number of posts per page (after disabling infinite scrolling), set your feed to provide summaries instead of full texts, and turn the blog to private or semi-private (or back to public), in Settings -> Reading.
• You add or remove sidebar (or footer) widgets in Appearance -> Widgets.
• You add share and like buttons, and select where they should or shouldn’t show up, in Settings -> Sharing.
• You select comment-related options in Settings -> Discussion.
Note well: changing the principal discussion setting (i.e. whether you allow or don’t allow comments) applies to posts and pages you create after you make this change. To change it on already published posts/pages, you need to edit those posts/pages; details here:
• You set the default widths for the thumbnail, medium and large image size options in Settings -> Media.
• You set your default post and link categories, and select the option “WordPress should correct invalidly nested XHTML automatically”, in Settings -> Writing.
• If you have the Custom Design upgrade, you make CSS changes in Appearance -> Custom Design.
[*] Very rare exception: on a few themes the Theme Options are hidden in Appearance > Themes > Customize.