Note: Unless you are in a Pro space using Themes, you can only change the style of one object at a time. If you select multiple objects, the right panel will switch back to displaying diagram details/comments.
Activity style, line style, free text style
First you need to open up the right panel and select an object (activity box, line, free text), and move the "Override selected theme style" slider to access the color settings. You can then change any styling on the object.
This includes text format, font, justification. Note: to format fonts or justify text you can select the whole box or just select specific text. The formatting in the panel then applies to that which is selected.
If you select a line, you can then select “Line form” in the right panel and select the line shape, type, and start and end point styles.
This will also allow you to create and manage basic Entity Relationship Diagrams (ERD). See complete article on using Elements for ERD mapping.
Copy & paste style to clipboard
COPY THEME and PASTE THEME allows you to add the style of one object onto the clipboard and apply it to another.
The style you have copied stays in the clipboard even if you go to other diagrams and other maps. If you then select "paste theme", it will remember what you had copied there and will automatically toggle that object to “Override selected theme” on.
Note: this is a setting for that given object. If you are applying themes (as below) across all objects, Elements will overwrite any object which has NOT had “Override selected theme” toggled "on".