Dear AWilderbeast,
After you set property "ThemeType" to "Custom", it will read styles from folder "CuteSoft_Client\CuteEditor\Themes\Custom\". If you remove "CuteSoft_Client/CuteEditor/Themes/Custom/dialog.css" and "CuteSoft_Client/CuteEditor/Themes/Custom/style.css", clear cache, and then run cuteeditor, you will find the whole style will be changed. dialog.css controls the style of dialog, you can click button "Insert Image", the dialog style will be changed.
Thank you for asking