Lookup[NotebookInformation[nb], "StyleDefinitions", {}]

processSheet // Clear
processSheet[{nb_NotebookObject, {implements_NotebookObject, ___}}] :=
   implements;
processSheet[_] := Nothing;
getSheet // Clear;
getSheet[nb_NotebookObject] :=

  processSheet@Lookup[NotebookInformation[nb], "StyleDefinitions", {}];
getSheet[e_] := e;
getStyleSheetChain[nb_NotebookObject] :=
 Rest@
  Module[{s = <||>},
   FixedPointList[
    getSheet,
    Replace[Lookup[NotebookInformation[nb], "StyleDefinitions", {}],
     {
      {n_NotebookObject} :> n,
      {n_NotebookObject, {i_, ___}} :> i
      }
     ]
    ]
   ]

mergeStyles[styles_]:=
  Merge[
    {
      styles
      },
    If[AnyTrue[#, ListQ[#]&&OptionQ[#]&],
      Merge[Flatten@#, Last],
      Last@#
      ]&
    ];
mergeStyleSet[styles_]:=
  Merge[
    Normal/@styles,
    mergeStyles
    ];

getStyleChain[nbs:{__NotebookObject}]:=
  Module[
    {
      cells=Cells/@nbs,
      names,
      ops
      },
    ops=ConstantArray[<||>, Length@nbs];
    MapIndexed[
      Replace[
        #,
        {
          Cell[StyleData[name:_String|All, env:_String|None:None], o___?OptionQ]:>
            Set[
              ops[[#2[[1]], Key@If[env===None, name, {name, env}] ]],
              Merge[
                {
                  Normal@Replace[
                      ops[[#2[[1]], Key@If[env===None, name, {name, env}]]],
                      Except[_Association]:><||>
                      ],
                  o
                  },
                Last
                ]
              ],
          c:Cell[
            StyleData[
              name:_String|All, env:_String|None:None,
              StyleDefinitions->StyleData[parent_]
              ],
            o___?OptionQ
            ]:>
            With[
              {
                inherited=
                  mergeStyles@
                    Flatten@List@Lookup[ops[[;;#2[[1]]-1]], Key@parent, <||>]
                },
              Set[
                ops[[#2[[1]],Key@If[env===None, name, {name, env}] ]],
                Merge[
                  {
                    Replace[
                      ops[[#2[[1]], Key@If[env===None, name, {name, env}]]],
                      Except[_Association]-><||>
                      ],
                    o
                    },
                  Last
                  ]
                ]
              ]
          },
        1
        ]&,
      NotebookRead/@cells
      ];
    mergeStyleSet@ops
    ]

getStyleSheetChain[EvaluationNotebook[]]

styles = getStyleChain[getStyleSheetChain[EvaluationNotebook[]]];

styles["CodeText"]

<|CellMargins -> {{66, 10}, {3, 15}}, LineSpacing -> {1, 1, 1},
 MenuSortingValue -> 1540, FontColor -> GrayLevel[0.45]|>