How can I easily format headings, terms, theorems, etc?
You can set paragraph margins, paragraph alignment, font face, font style, and font size individually for a paragraph or selection, but it can be tedious to set these values repeatedly in order to make section headings, theorems, display math expressions, examples, etc. have a consistent look throughout your document. To avoid that you can define named styles in Lurch that store one or more of the above five formatting settings so that the entire collection can be applied to a paragraph or selection easily in the future.
To do so simply create an example of the style you want to define in your document, and then with your cursor in the sample, use the Format>Manage formats menu to open a dialog that lets you define a named formatting style based on your current cursor position. After that you can apply that formatting style to a paragraph or selection by pressing CTRL+SHIFT+F and choosing the style from the list. Pressing CTRL+ALT+SHIFT+F applies the last style you selected, which is helpful when you have to apply the same style to many different selections or paragraphs.
Note that applying a named style in Lurch only applies the attributes of that style to the selection or paragraph. It does not store the name of the style itself, so that if you redefine the style later on it will not affect the formatting of sections of the document to which that style was previously applied. A collection of styles can be saved as a Lurch document in your Lurch search path (see File>Preferences to manage and identify search paths) and then used as a dependency to act as a style sheet for your documents. Lurch comes with several such Stylesheet documents that you can add to your own as a dependency (see File>Document Properties to manage your dependencies).