Optimize font sizes and layout

[vkrasnovyd]

Closes #155

1. Font sizes

• Made font size depend on the screen size. Adjusted the ranges of possible sizes so there's more balance between the fonts of different font sizes;
• Reduced line height for the header on 'Tools' page.

2. Layout updates

• To make layout more consistent, changed text alignment in 2 blocks on the start page (header and standalone sentence before the video) from 'center' to 'start'. Reduced vertical margins for them to reduce their visual weight;
• On the main page applied the same width limit to the header as for the rest of the page.

[bernhardreiter]

LGTM & works.

Looks much better, see the same simulated laptop (compare to #155)

Also visible changes are in the tools section and in second header on the front page.

We use left alignment instead of centering everything, this usually gives the better usability (though might be make people that know the old alignment stumble at the first view.)

[bernhardreiter]

@justindmurphy or @tschmidtb51 I suggest to accept the change and close #155.