Changing color, format and themes for different parts of diagrams might help you with visualising parts of processes, differentiate departments responsible for certain actions and more.

First you need to select the flow line, right-click and from the menu pick "Format line theme" to open up the right panel and move the "Override selected theme style" slider to access the settings. You can then change any styling on the object.  
This includes text format, font, justification.

TIP 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.

TIP This will also allow you to create and manage basic Entity Relationship Diagrams (ERD)

"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.

TIP 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".

TIP 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.