Twenty Fifteen: H5 and H6 headings have the same font size

[poena]

This was originally reported by a user via the report button in the theme directory:

there is one 'elements' issue, namely, Header 6.
When using Header 6, it appears to be as the same size with Header 5 (both same size?)

-Confirmed that both H5 and H6 headings have a default font size of 17px.
Headings should be styled to indicate their importance, where a lower heading level has a smaller size.

Because the default paragraph font size is also 17px, a solution would be to increase
the font size of the H5 heading.

[akabarikalpesh]

I added h5's font size is 18px. let me know if any changes.

[hellofromTonya]

The styling in the theme was introduced in 4.1 ​in this commit.

[afrid1719]

I have increased the font-size for H5 by 1px for all devices having screen size greater than or equal to 740px.