I want to enable the end user to customize the syntax highlighting of the Monaco editor. I know there's defineTheme but I want individual functions for each part of the Monaco colors like the code I have below.

SetBackground = function(background) {
                monaco.editor.defineTheme('Custom', {
                base: 'vs-dark',
                inherit: false,
                rules: [],
                colors: {
                    'editor.background': `#${background}`,
                }
                });
            }

This code works for setting the background but when I use my other function for setting the foreground the background reverts to that of vs-dark. Is there any way to write functions like these that add on to an existing theme and update it using user input?