Easy to use customizations for the default theme Twenty Eleven--add sidebars back into your blog pages, and quickly change individual color settings.
No, the plugin has been designed to work specifically with Twenty Eleven, the current default WordPress theme, and child themes using Twenty Eleven as their template. Most of the features depend on the HTML and CSS used by this theme, and will almost certainly not work with the vast majority of the other themes out there.
Yes, you can. The plugin checks to see if the current theme is a child of "Twenty Eleven" and also looks for the phrase "Twenty Eleven" in the name of the theme, so as long as it is a child theme of "Twenty Eleven" or the theme's name (obtained from the styles.css file in the theme directory) contains "Twenty Eleven", the Theme Extensions admin page will be active and the extensions will be available to you.
Please note, however, that some or all of the plugin's features may not work with your customized version of Twenty Eleven. If you, other plugins or child themes have changed something this plugin depends up, then the results cannot be guaranteed. But the only way to tell if it will work is to give it a try. If something doesn't work, you can easily disable the option again on the administration page.
Not all the colors used in Twenty Eleven are configurable from the Theme Options page, but you can still use the Theme Extensions plugin to change those that are not listed.
Look up the CSS styling you need to modify in the styles.css file of Twenty Eleven. (Click on Appearance >> Editor and locate styles.css near the bottom of the list of files on the right.)
Now enable the Custom CSS option on the Theme Extensions admin page, and enter the CSS styles you want set for your blog.
If you have a lot of changes to make, it would be better to create a child theme, and put the CSS changes in its own styles.css file. For more information, see the WordPress Child Theme documentation.
Finally, you can add your own color settings using a filter called by the Theme Extensions plugin. For more information, please see Adding Your Own Custom Color Settings.
There is no automatic way to recrop existing header images to make them smaller. You can turn on the option to force all images to the new height, but that will distort those images that are taller or shorter than the new height, which will include all of the preinstalled header images that came with the theme.
The option is really intended to allow you to upload new images (or new copies of existing images) that will be cropped to the new height. Therefore, if you like the set of header images that comes with the theme, but want to use smaller versions of them, you will have to download the images to your computer (right click on them and select "Save Image As") and then use the Header theme options page to upload them at the new height. This will allow you to crop the image just the way you want it.