[CSS] Font color contrast

[graceguo-supercat]

I noticed there are some css and styling consolidation recently. It results some gray text become much lighter. See Run Query and other buttons:

Screenshots

In the current font color standard, it's hard to tell the button is active or inactive state. And color contrast is not strong enough it makes me feel hard to read (but probably just me 😢

Environment

(please complete the following information):

• superset version: superset version
• python version: python --version
• node.js version: node -v
• npm version: npm -v

Checklist

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• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

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[rusackas]

Heya @graceguo-supercat :)

I'll make some touchups to address these issues. There were some cases where shades of gray were rounded to the nearest level a bit more harshly than others - I'll adjust the lighter bits of UI seen here.

We may find some additional elements that need tweaks, so please feel free to add to this PR or ping me on Superset Slack, and I'll tackle them as they come up.

[rusackas]

@graceguo-supercat Just opened #8650 to address this.

[graceguo-supercat]

Verified it is fixed in the latest build!