Builder, Custom CSS are not being applied

Having an issue with styling Post Titles in Builder - in this instance specifically for mobile. When I click the cog at the top of the Posts box. I go to Appearance where I have set up a new Preset called Mobile which I have styled to bring the font size down. When I Save, Update then Save/Export and two things go askew: Firstly the style does not seem to get updated to another browser when viewed. Secondly when I exit Builder and go back in again the Preset under Appearance for mobile has reverted back to "Default".