customizable design

[taschenuhr]

In addition to #149, this PR contains options to customize the design

• Define a local or publicly available SVG, that will be displayed as logo
• Reference a local CSS file to apply additional CSS rules

[cupcakearmy]

thanks for the contribtion. since you arlready have logic for detecting an url or not, i would suggest not creating a new variable for svg, but just reusing the image one. make it accept an svg in base64 format. wdyt?

[taschenuhr]

Thank you for the feedback. You're right, we don't need a second configurable environment variable. And base64 encoded input is great, too.
Both is done now.