Improve syntax highlighting

[m-rossi]

Syntax highlighting is working since #8/#40, however not all keywords are highlighted as you would expect it/see it in Jupyter. This is especially true für the import keyword. The reason is the default color scheme pygments. We need to provide an edited theme, for example this definition colors the import statements:

"Import": {
    "text-color": "#008000",
    "background-color": null,
    "bold": true,
    "italic": false,
    "underline": false
},