Discover table disappears when dragging fields from the field list (elastic#209714)

## Summary

Closes elastic#209442 

After dropping [color utility
functions](https://eui.elastic.co/#/theming/colors/utilities)
accidentally drag and drop zone background became solid, hiding table
content.

![image](https://github.com/user-attachments/assets/5c5b9686-ee11-4407-909f-56c53e4a0cd1)

EUI team agreed to revert `transparentize()` in this particular case
(until migration from SCSS to Emotions, when a new proper token should
already exist). Current color tokens though demanded a bit different
opacity values than previously. I was trying to achieve the most similar
effect to the `8.17` version.

I also adjusted border as it was a bit too light compared to `8.17`.

**8.17 border:**
<img width="176" alt="Screenshot 2025-02-05 at 10 43 35"
src="https://github.com/user-attachments/assets/bf71747a-63a5-4630-8a70-674a9576dbe0"
/>


**`main` border**
<img width="181" alt="Screenshot 2025-02-05 at 10 34 39"
src="https://github.com/user-attachments/assets/c0756f20-ae74-4c72-914c-3b9fde96faa0"
/>

**My choice from existing tokens (there's nothing in between `base` and
`strong` success for borders):**
<img width="174" alt="Screenshot 2025-02-05 at 10 42 24"
src="https://github.com/user-attachments/assets/584f5577-4842-4b44-a395-3b0bcbaf4c82"
/>


Below you can find gifs with both versions in action. I'm open to any
other color-opacity pairs suggestions if those don't mirror the previous
state enough.

**8.17**
![8 17 drag and
drop](https://github.com/user-attachments/assets/a95db5da-1af1-4373-a58d-6ecda33f9387)

**This PR**
![PR drag and
drop](https://github.com/user-attachments/assets/cf6c1a1e-8f8d-4462-8984-88837289ccd1)


### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

~~- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)~~
~~- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials~~
~~- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios~~
~~- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)~~
~~- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.~~
~~- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed~~
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

### Identify risks

Does this PR introduce any risks? For example, consider risks like hard
to test bugs, performance regression, potential of data loss.

Describe the risk, its severity, and mitigation for each identified
risk. Invite stakeholders and evaluate how to proceed before merging.

- [ ] [See some risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx)
- [ ] ...

---------

Co-authored-by: Elastic Machine <[email protected]>

Author: akowalska622

src/platform/packages/shared/kbn-dom-drag-drop/src/sass/drag_drop_mixins.scss

@@ -39,18 +39,20 @@ $domDragDropZLevel3: 3;
 }
 
 // Style for drop area when there's an item being dragged
+// Until SCSS -> emotions migration we should continue using "transparentize()" here
 @mixin mixinDomDroppableActive($borderWidth: $euiBorderWidthThin) {
-  background-color: $euiColorBackgroundBaseSuccess !important;
+  background-color: transparentize($euiColorBackgroundLightSuccess, .7) !important;
   &:before {
-    border-color: $euiColorBorderBaseSuccess !important;
+    border-color: $euiColorBorderStrongSuccess !important;
   }
 }
 
 // Style for drop area while hovering with item
+// Until SCSS -> emotions migration we should continue using "transparentize()" here
 @mixin mixinDomDroppableHover($borderWidth: $euiBorderWidthThin) {
-  background-color: $euiColorBackgroundLightSuccess !important;
+  background-color: transparentize($euiColorBackgroundFilledSuccess, .75) !important;
   &:before {
-    border-color: $euiColorBorderBaseSuccess !important;
+    border-color: $euiColorBorderStrongSuccess !important;
   }
 }