Hi,
If you wish to make global CSS changes your CSS definitions should be created on the highest level possible, typically these are applied to a Master Mashup that contains all of the sub mashups and thus the widgets you are trying to edit.
One thing to note is depending on what version of ThingWorx you are running, the widgets and HTML structure differs on what class you actually need to edit. Typically how I locate what classes need to be edited is I use Chrome Developer Tools while my mashups are being previewed, you can dynamically adjust the styling and see real time feedback of your edits, and this assists in locating what exact out of the box CSS class you need to override.
