A Text Style applies to all text objects in the score of a particular type. Staff text objects, for example, have a unique style, as do all tempo markings, all lyrics, all chord symbols and so on.
You can view and edit all text styles by selecting Format→Style…→Text Styles. This gives you access to the following text properties for each style:
Border radius: For box frame, radius of rounded corner.
Notes: (1) Opacity is set by the parameter "Alpha channel" in the "Select Color" dialog: a value between 0, transparent, and 255, opaque. (2) Other text properties, such as font color, can be set in the Inspector.
You can also edit the text properties of a Text Style by selecting an object of the desired style in the score, adjusting its properties in the Inspector, and clicking the Set as style buttons as you go. This will automatically update the Text Style and all relevant text objects in the score.
If you have made changes to a particular text property in the Inspector, but want to revert that property to the Text Style definition, press the "Reset to style default" button.
Text Styles (together with all other styles in a document) can be saved as a style file and loaded into other MuseScore files. See Save and load style.