jupyterlab/style/sources.css

[data-jp-debugger='true'].jp-Editor .jp-mod-readOnly {
  background: var(--jp-layout-color2);
  height: 100%;
}

.jp-DebuggerSources-body [data-jp-debugger='true'].jp-Editor {
  height: 100%;
}

.jp-DebuggerSources-body {
  height: 100%;
}

.jp-DebuggerSources-header > div > span {
  overflow: hidden;
  cursor: pointer;
  text-overflow: ellipsis;
  white-space: nowrap;
  font-size: var(--jp-ui-font-size0);
  color: var(--jp-ui-font-color1);
}