When you create a text object, it initially assumes the properties of the Text Style applicable to its class. Staff text objects, for example, have their own style, as do tempo markings, lyrics, chord symbols, and so on.
To view and edit any Text Style: select Format→Style…→Text Styles. This gives you access to the following properties:
Corner radius: For box frame, radius of rounded corner.
Notes: Opacity is set by the parameter "Alpha channel" in the "Select Color" dialog: a value between 0, transparent, and 255, opaque.
Alternatively, you can edit the 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.