Ensure sg_theme function implements charts guidance

[lee-bunce]

Ensure that the sg_theme function fully implements the recommendations of the GSS Charts guidance.

For example:

• Use appropriate font size
• All text should be horizontal
• tick marks
• grid lines
• gaps between bars
• etc.

[alice-hannah]

• [x] Axes and axes labels: #595959
• [x] Grid lines: #D9D9D9
• [x] Right align values on y axis
• [ ] Use comma as thousands separator?
• [x] All text horizontal (if difficult to read, consider different orientation of chart)
• [ ] Categorical data, labels should be between tick marks

[alice-hannah]

I've done my best to apply as many of the guidance recommendations as possible.

However, I don't think there's a sensible way of controlling the following for all charts:

• Size of gap between bars (https://r-graphics.org/recipe-bar-graph-adjust-width)
• Positioning of axis labels between axis ticks (don't think this can be done in ggplot2)
• Use comma as thousands separator in chart labels/text

I've added these to the list of considerations in the accessibility article so they don't get lost/overlooked.

Please reopen this issue if you are reading this and have any ideas!