Overhaul CSS in performance bar in Dark mode v1

What does this MR do?

Overhaul CSS in performance bar in Dark mode to make these text visible to mainly fix the regression happened from !52775 (merged) (%13.11).

• color: #999 ($gray-300) + #fff ($gl-text-white) + #dbdbdb ($gray-100) => #dbdbdb ($gray-100)
• background: many duplications => eliminated

Most of them has not been changed after gitlab-foss!11439 (merged).

Screenshots v2

env	before	after
dev		(selection is just artifact)
stg	(omitted)	(simulated)
prd		(simulated)
select/option box (dev) on Chrome	-
select/option box (dev) on Chrome (pull-down)	-	​
select/option box (dev) on Firefox	-
select/option box (dev) on Firefox (pull-down)	-

Screenshots for avoiding regressions

Screenshots in Indigo theme:

env	after
dev
stg
prd

Screenshots v1

env	before	after
dev
stg	(omitted)	(simulated)
prd		(simulated)

Screenshots for avoiding regressions

Screenshots in Indigo theme:

env	after
dev
stg
prd

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.

Closes #330702 (closed)