@morosirus,

The font size drop-down in the formatting menu.

The CSS code below should help you to solve the issue: 

.mce-menu .mce-container-body {
    background-color: #2f2c2c !important;
    color: #fff !important;
}

The tag auto-fill drop-down in the tag line.

Use this CSS code: 

ul.wpf_ac_results {
   background-color: #2f2c2c !important;
   color: #fff !important;
}

The CSS codes should be added in the "Custom CSS code" textarea, located in the Dashboard > Forums > Settings > Styles Tab. 

The red marked value can be changed as you like.   

Don't forget to press Ctrl+F5 (twice) on the forum frontend before checking to reload the updated CSS file.   

Seemed to do the trick. Thank you.