Note: the edittable plugin doesn't respect styles.
The actual problem is, the styles that are not suitable for you are set for the plugin. You can format the editor window of the plugin via CSS as well as all other contents of DokuWiki. Presumably it is enough if you switch the editor background from transparent to white.
In addition, I recommend this plugin: https://www.dokuwiki.org/plugin:tablewidth
HTH - Michael Sy.