[Dark Theme] Text Colours for Titles and SideBar

[AakashGfude]

This PR introduces the following colour:

Please refer to the issue https://github.com/QuantEcon/quantecon-book-theme/issues/235

fixes https://github.com/QuantEcon/quantecon-book-theme/issues/235

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e65778b) 73.18% compared to head (05b8cb2) 73.07%. Report is 5 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #241 +/- ## ========================================== - Coverage 73.18% 73.07% -0.11% ========================================== Files 2 2 Lines 261 260 -1 ========================================== - Hits 191 190 -1 Misses 70 70 ``` | [Flag](https://app.codecov.io/gh/QuantEcon/quantecon-book-theme/pull/241/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=QuantEcon) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/QuantEcon/quantecon-book-theme/pull/241/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=QuantEcon) | `73.07% <100.00%> (-0.11%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=QuantEcon#carryforward-flags-in-the-pull-request-comment) to find out more.

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[mmcky]

thanks @AakashGfude

[mmcky]

thanks @AakashGfude when you are ready please make a release 0.7.1