Rollup merge of #75837 - GuillaumeGomez:fix-font-color-help-button, r=Cldfire
Fix font color for help button in ayu and dark themes
A nice before/after:
![Screenshot from 2020-08-23 14-47-07](https://user-images.githubusercontent.com/
3050060/
90979230-
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![Screenshot from 2020-08-23 14-47-03](https://user-images.githubusercontent.com/
3050060/
90979233-
145eeb80-e554-11ea-8e63-
1864c3f2699b.png)
For the ayu theme, the change is very "light", the font color was already close to white, so I unified the color with the pictures of the other buttons:
![Screenshot from 2020-08-23 15-20-45](https://user-images.githubusercontent.com/
3050060/
90979281-
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![Screenshot from 2020-08-23 15-20-50](https://user-images.githubusercontent.com/
3050060/
90979279-
5daf3b00-e554-11ea-8d39-
beb57091aba7.png)