You may use in WordPress a Twitter share button without any problem until recently, when you found out that suddenly the text in your Twitter button is white, almost invisible.

This is not a Twitter, nor a problem of your plugin. Most probably you installed in your browser an extension that defined the .btn class as white. This is the class used also by Twitter to define the color of its button text.

To solve this problem you can uninstall your browser extension, or define in your css file a .btn-default class and give it the color value you like, such as black, grey, navy, teal, etc. Do not forget to add the !important direction to make sure your definition acquires priority. Here is an example of a full entry you can use in your stylesheet.

.btn-default { 
color:teal!important;
 }