In your theme folder > styles > sass > lib > components there is a file called "dataview.scss" where the styling for dataviews is maintained.
Looks like the styling for the control bar is held in a class called ".mx-dataview-controls" and the background color is using a variable called "$dataview-controls-bg". You could change that variable to transparent if you want to make the color transparent for all dataviews or you can create a custom class to apply to specific dataviews where you dont want the grey background.
If you make any changes in the sass files dont forget to recompile them. Heres a link to the documentation for setting this up with koala.
https://docs.mendix.com/howto/ux/setup-mendix-ui-framework-with-koala
like Austin said,
It looks like something is messing with the dataview styling.
there is also this guide on how to use sass with gulp.
https://medium.com/@jasonteunissen/how-do-i-start-styling-in-mendix-gulp-sass-6b37ddaf8de6