When the text box is selected, the following styles can be edited in the "Text" tab of the style bar.
Size / Thickness
The font size is specified in points.
Depending on the font, you can also change the weight.
Letter spacing / Line-height
The character spacing is specified in "em", where 1em is the standard character spacing, and the larger the number, the wider the spacing between characters. In "Line Height", you can set the height per line of text.
Italic / Underline
You can italicize or underline your text.
You can specify the color of the text by opening the color panel. You can't select a gradient here.