Three drop-down menus at the top of the Rich Text Editor window allow you to control the appearance of objects:
1. Block format
This allows you to choose between HTML styles such as paragraph (the default) and various headers. Simply place the cursor within a block of text and select a format.
2. Block style
This allows a particular style to be applied to an object. The styles available are different depending on the type of object selected, i.e. whether it is a paragraph, an image etc. For paragraphs you can choose, for example, to centre the text or to apply a colour. For images you can choose to align the image to the right, to the left etc.
3. Text style
This allows you to apply a style to a piece of text within a paragraph. For example, this text uses the "Coloured" style. Simply select a piece of text then select the style.
Note that it is possible to apply several styles to the same piece of text. This is legitimate but is probably not what you want. If changing from (say) the Fixed Width style to the Coloured style you first need to click on the text and select Remove Text Style before setting the new style.