Yes, the button text can be customized. Please refer to the following topic that lists the available string resources used for buttons and how to change them.
There is currently not a way to set a maximum size for ThemedMessageBox directly. Ultimately, ThemedMessageBox is just a wrapper around UserPrompt and UserPromptWindow. It just configures UserPrompt in a way that is consistent with native MessageBox and provides a convenient binary compatible API with native MessageBox to allow code to easily transition. For more control over the resulting dialogs, you could write your own wrapper that configures and displays a UserPrompt.
If you can wait for the next maintenance release, we have improved handling of large text so that the width automatically increases in logical steps until it is wide enough to display the content on the screen without exceeding the available height. This is similar to how the native MessageBox behaves. If still too tall at the maximum width, a vertical scrollbar will be used for the content to avoid making the window too large (something the native MessageBox does not do).
Thank you for bringing the width issue to our attention.