Use light grey for siteNav text

[Xenonym]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] Enhancement to an existing feature

Resolves #542.

What is the rationale for this request?
We can have lighter coloured text for the site navigation sidebar to give more relative importance to the main text.

What changes did you make? (Give an overview)
Adjust the colorfor siteNav text to be #444444.

Testing instructions:

1. Observe that in MarkBind documents with a navigation sidebar, the colour of the text is #444444.

[damithc]

A little bit lighter? See https://hexo.io/docs/ for an example.

[Xenonym]

@damithc actually, I based it off Hexo; #444444 is the color of .sidebar-link. We could use the built-in Bootstrap grey, which is #6c757d. Here's what that looks like:

Alternatively, we can do the middle ground of #565656 for better contrast:

[damithc]

@Chng-Zhi-Xuan what do you think?

[Chng-Zhi-Xuan]

Looks ok to me. If you would like some accent colour, we can keep the blue on the rotating arrow when it is active?

Edit: Forgot to mention #565656 strikes a good balance for pure white backgrounds, Am ok with #444444 as well.

[Chng-Zhi-Xuan]

Make the changes, squash any excess commits and force push for final approval.