You can make minor changes to the look and feel of a theme from within WordPress.

Note: Before making any changes to a WordPress theme, create a child theme to avoid losing your changes when updating the theme.

Login to your WordPress admin area. 
Click Appearance.
Click Customize.
From here, you can make changes to the following parts of the site:

        Site Identity
        Layout
        Colors
        Fonts
        Header Image
        Background Image
        Menus
        Widgets
        Static Front Page

Note: The options available will depend on what the author of the theme has made available.