More Settings in Customizer to target other defaults not listed

Hi All,
How can I add or get more settings into the customizer in order to target some presets that would need css override rules.

Specifically, for example, I would like to have something under the General tab.
One for targeting blockquote and another to target input fields.
Both these seem to grad some defaults of the LESS files. Easy enough for me to adjust them with CSS, but painful. Also would be easier to simply adjust them within the customizer like most other themes.

Looking at block quotes … Imagine you can just add that to General , where we can sets the left border color and width and a block background color if we choose not to use the default variable @border-color which.
For input fields that are pulled into forms etc, some settings specifically to set border colours and placeholder colours.

This doc for adjust seems like a great start, but it is missing these particular items that comes up with EVERY new site build.

Any help would be great thanks.