@jupyter-lsp/jupyterlab-lsp/style/signature.css

Language Server Protocol integration for JupyterLab

.lsp-signature-help code {
  font-size: var(--jp-code-font-size);
}

.lsp-signature-help code > mark {
  color: var(--jp-lsp-signature-mark-color);
  background: var(--jp-lsp-signature-mark-background);
}

.lsp-signature-help code > mark > span {
  /* override syntax highlight colour to ensure contrast */
  color: var(--jp-lsp-signature-mark-color) !important;
}