I have downloaded OS MasterMap stylesheets from OS/Github.
How do I apply them in QGIS, all I get is style code rules for each symbology? see below picture example from Topo area file.
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Your screenshot looks like you're most of the way there, the Rule column is looking at the
style_code field and saying every feature with the attribute of 1 will be displayed as a Building.
Of course, this assumes you have data in the correct format and has the relevant fields in the attribute table - which it should be if it has been provided by the OS, but is not clear from your screenshot.
Take a look at this doc from the OS for more info:
It goes through the steps required to apply the .QML files (stylesheets) from GitHub to your data, and will hopefully clarify if you're still having issues.