Increase submission modal title contrast

[raicem]

Fixes #752.

Props bor0, akeda

How to test the changes in this Pull Request:

1. Go to https://wordpress.org/patterns/new-pattern/
2. Add at least one block
3. Click Submit to get the modal show

Before:

After:

[bor0]

Because of #754, I had to test this on my local machine instead of the sandbox. Make sure Docker is running and follow:

bor0:~/dev/pattern-directory$ git fetch origin pull/753/head:pr-753
bor0:~/dev/pattern-directory$ git checkout pr-753
bor0:~/dev/pattern-directory$ nvm use
bor0:~/dev/pattern-directory$ npm install
bor0:~/dev/pattern-directory$ npm run create # had to create and supply a GH token...

I then visited http://localhost:8888/ and confirm it worked well!

[bor0]

Post-merge deploy:

1. Wait for https://github.com/WordPress/pattern-directory/actions/runs/28582399529/job/84745618628 to complete

2. On sandbox, first cd /home/wporg/public_html and then svn up and then bin/sync/pattern-directory.sh

Authentication realm: <https://dotorg.svn.wordpress.org:443> Use your WordPress.org login
Enter SVN Username: bor0
Password for 'bor0': 

Syncing to SVN...

Changes:
M       /pattern-creator/build/style-index-rtl.css
M       /pattern-creator/build/style-index.css
M       /pattern-creator/src/components/submission-modal/style.scss
M       /style.css

Press d to view diff (q to quit diff mode), any other key to continue. 

3. Now everything is committed. Just deploy with deploy-dotorg.sh wporg

Fix is live on https://wordpress.org/patterns/new-pattern/