Display and Modify CSS variable ? (Module)
You have many options:


Make a system settings form that stores the value to a drupal variable. Read that value from your (themes) css. Then when that form is submitted in your theme form submit handler have code clears the cache:   cache_clear_all("{$theme}:css:*", NULL, TRUE);. Where $theme is your theme name. The CSS should be re-generated on page load.
I believe the LESS module allows you to easily make CSS variables applied to your files (in Less-3.x or greater for drupal). If you're not currently using LESS, then this is less helpful (pun NOT intended!)