On the font problem, one easy way to improve readability would be to go from the current Roboto Light (font-weight: 300; ) to Roboto Regular (font-weight: 400; ) - it's not a radical change in overall word/paragraph size but it does improve readability especially in Chrome. In Firefox either font weights work since anti-aliasing is more crisp, while Edge also favors 400 over 300 as well.
Also, on the Dark Theme, you might want to consider moving from a pure white font in posts (#fff) to a very light gray (#eee or anything between this and #fff). Pure white text on dark surface tends to be more tiresome to read than pure black on light surface, so in order to make the most of a dark theme in terms of eye strain one can slightly adjust font brightness as well.