Visual Studio Code Integration theme support

[akera-io]

The VSCode integration already uses VSCode CSS variables for sprotty graph/node/edge, problem is with text which remained the default color (black). Depending on the theme selected some of the text in the diagram is almost not visible - either labels inside the nodes or on the edges.

Override fill color for sprotty text in diagram.css:

/* most of the time the labels are inside the nodes */
.sprotty text {
fill: var(--vscode-editor-background);
}

/* for edges background is the graph: --vscode-editor-background */
.sprotty-edge text {
fill: var(--vscode-foreground);
}

[martin-fleck-at]

@akera-io Thank you for reporting this issue! In general this change looks reasonable to me, would you like to open a PR for it so we can do a proper review on it?