You can now set the opacity value of an entire element, in addition to setting the opacity (or alpha value) of a color. This value can be set as a part of a Style, or individually on an element. A value of 100 means fully opaque and a value of 0 means fully transparent. The default for all elements is 100. This new ability gives you additional control over the look and feel of your application, and is compatible with any opacity animations you set on your element!
Read more documentation here: Styling Properties - Bubble Docs