You are unable to select the Web 2.0 theme for use with a local rendering portlet.
By default, the local rendering portlet is on the Web 2.0 theme's portlet blacklist because of a known issue where the browser applet used to edit content with Rich Text fields does not load correctly in Internet Explorer when using authoring tool components.
If this is not a valid use case in your environment, then you can choose to remove the portlet from the blacklist at your own discretion. It is recommended that you test the local rendering portlet in the Web 2.0 theme before putting it into production.
Resolving the problem
To use the Web 2.0 theme with a local rendering portlet, you need to remove the local rendering portlet from the blacklist by either removing or commenting out the following line from the portlet blacklist, or by setting the value to "false":