[FEATURE] add broader themes

Currently we can define a theme to set specific attributes of specific models like below from the examples in https://docs.bokeh.org/en/latest/docs/reference/themes.html

attrs:
    Figure:
        background_fill_color: '#2F2F2F'
        border_fill_color: '#2F2F2F'
        outline_line_color: '#444444'
    Grid:
        grid_line_dash: [6, 4]
        grid_line_alpha: .3
    Title:
        text_color: "white"

Would it be possible to add something a bit more powerfull like matplotlib’s rcParams to set attributes like text properties for all the text in the document. For example with matplotlib we can do rcParams.update({'font.size':15}) and the font size of all text, from titles, to axis labels, to legend labels, is updated.

Alternatively I loop over all the models I know in the document that have text, look at their .properties_with_values() dictionary, and build the theme dictionary by looking for all the attributes that include e.g. “text_font_size”.

[bryevdv]

Thinking about this also from the other issue wanting to target themes to named objects, I might suggest this instead:

# theme by mode type
attrs:
    Figure:
        background_fill_color: '#2F2F2F'

# theme by model name
names:
    # ignore values when they don't apply to a model with the name?
    major_label_orientation: 0.4

# theme by property kind
props:
    text_font: "arial"

I don’t support further subdividing attrs There is no point it having attrs in the file at all if it is the only valid top level construct. I’d rather split things at the top level, and also avoid weird syntax with * etc

[mattpap]

I consider this as completed based on PR #9755. If that’s not the case, please reopen.