apply_theme() is used to apply themes to your program. Its syntax is as follows:
CALL apply_theme(theme_file)
where theme_file stands for a string value identifying the name of the theme to be applied:
CALL apply_theme("my_styles")
When you specify a file name, be careful with its case.
UNIX is case-sensitive, so it will treat files which names come in different cases as separate files.
Some of the properties are applied to the application right after its starting. These properties won't have effect if they are added or changed at application runtime.