Fix area style opacity for deployment frequency forecast

What does this MR do and why?

Fixes area style opacity for the deployment frequency forecast

• Minor refactor base settings into a constant
• Fixes the opacity level to match the deployment frequency chart data series
• Fixes both for light mode and dark mode

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before - light	After - light	Before - dark	After - dark

How to set up and validate locally

You will need to enable 2 feature flags

dora_charts_forecast # group
use_holt_winters_forecast_for_deployment_frequency # per project

1. Seed data for your project

2. Enable dora_charts_forecast for the group

   Feature.enable(:dora_charts_forecast,  Group.find(GROUP_ID))

3. Enable use_holt_winters_forecast_for_deployment_frequency for the project

   Feature.enable(:use_holt_winters_forecast_for_deployment_frequency,  Project.find(PROJECT_ID))

4. Browse to http://{local-instance}/{project-path}/-/pipelines/charts?chart=deployment-frequency

5. Click the toggle

6. Toggle dark mode to verify

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #421272 (closed)