Using Forms we have to add our own Styles to it. As in 12 the front end mark up on the controls are no longer available we have to concentrate more in inspecting the Markup and adding classes into our styles css file. Will Umbraco change these existing classes or is there a better way to manage to the styles?

You could create your own theme items for the parts you want to be custom

Also you can download the default theme files here: https://our.umbraco.com/packages/developer-tools/umbraco-forms/ Scroll to the bottom, select the "Source code" tab and pick the package that fits your version - seems not all are there, so they probably only update when changes occur.