General developer forum

PHP is the primary language used for Moodle development, so if you're looking to customise Moodle you'll be needing to write some PHP.  More recent additions to Moodle are using mustache templates to generate HTML output, so you can customise those parts by just overriding the the template.

You should start be reading the Themes documentation to get an understanding of how you customise Moodle's appearance.  Once you understand the basics of renderers and creating overrides in your theme, posting here or in the Themes forum with specific questions is a good way to get help.