We develop an effective low-frequency theory of the electromagnetic field in equilibrium with thermal objects. The aim is to compute thermal magnetic noise spectra close to metallic microstructures. We focus on the limit where the material response is characterized by the electric conductivity. At the boundary between empty space and metallic microstructures, a large jump occurs in the dielectric function which leadshen to a partial screening of low-frequency magnetic fields generated by thermal current fluctuations. We resolve a discrepancy between two approaches used in the past to compute magnetic field noise spectra close to microstructured materials.