the inline style always overrides the external and internal CSS, and the fact that the plugin uses the !important clause (very bad practice!) all together makes it impossible to fix it only with css. I believe that you will have to use some custom js to override the plugin settings.
perhaps the best way is to check if you can specify a callback function with the plugin and set the style as you like. another answer here suggested editing the plugin itself, which is great if you do not plan to update it, otherwise you better leave the plugin code as is, and just add some special js of your own
Luca
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