H2O gradio theme

[jefffohl]

This PR does the following for the Gradio app:

• Adds H2O logo to header
• Creates an H2O theme, using H2O brand colors
• Improves accessibility by adjusting colors for dark mode
• Tweaks a few things to make them line up better
• Eliminates the h2ocolors argument

If we need to retain the h2ocolors argument, please let me know, and I can add that back.

Revised UI looks like this:

[pseudotensor]

Ya I'd add h2ocolors back. This is an open source project and not everyone necessarily wants those colors. Indeed, I like the original cool blue colors.

[pseudotensor]

I'll fix now and clean-up so the svg code isn't inside generate.

[jefffohl]

OK - thanks @pseudotensor - let me know if I can help.