Hi,
Nowadays more apps allow users to change or apply custom themes. Some reasons to add support for theming are:
style="
--bs-primary: #3E97FF;
--bs-primary-active: #0095e8;
--bs-primary-light: #f1faff;
--bs-primary-inverse: #ffffff;
--bs-primary-rgb: 62, 151, 255;
--bs-text-primary: #3E97FF;
--bs-component-active-color: #ffffff;
--bs-component-active-bg: #3E97FF;
--bs-component-hover-color: #3E97FF;
--bs-component-hover-bg: #F9F9F9;
--bs-component-checked-color: #ffffff;
--bs-component-checked-bg: #3E97FF;
--bs-menu-link-color-hover: #3E97FF;
--bs-menu-link-color-show: #3E97FF;
--bs-menu-link-color-here: #3E97FF;
--bs-menu-link-color-active: #3E97FF"
bookmarking
Also, I want to add my two cents: https://webcapitan.com/blog/wordpress-vs-custom-cms/. It gave me a basic understanding of what web development is. I would say this is a solid foundation for your future self-development.