Depending on the SAP theme, custom themes can support various clients and frameworks.
Your theme is based on SAP Gold Reflection : (zsap_goldreflection/library.css).
But, you need to know that custom themes that are based on SAP Gold Reflection are mostly not supported by SAPUI5 mobile libraries. For this reason, there are no CSS files for this and hence the request fails.
However, you may use SAP Blue Crystal as a basis for your custom theme as it supports SAP Fiori.