Some CSS selectors in the styles.css file are not specific enough to the contents provided by the module. The defined styles may affect other parts of Moodle outside your plugin scope (once they are concatenated with other plugins). Typical example may be selectors like .addnewentry. It is recommended that the selectors are specific to the content generated by the module only - typically by using something like .path-mod-dataform .addnewentry. Note that you may find LESS or other CSS pre-processors useful for this kind of things.