Adds dark theme images for multi-GPU and actuators (#601)

# Description

Previously, the images were rendered as "white" even in the dark theme
of the documentation. This MR fixes the images and adds their
counterpart for dark theme.

## Type of change

- This change requires a documentation update

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have run all the tests with `./isaaclab.sh --test` and they pass
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

docs/source/features/actuators.rst

@@ -59,10 +59,17 @@ actuator model, such as a DC motor, would require configuring a different actuat
 
 The following figure shows the actuator groups for a legged mobile manipulator:
 
-.. image:: ../_static/actuator_groups.svg
-    :width: 600
+.. image:: ../_static/actuator-group/actuator-light.svg
+    :class: only-light
     :align: center
-    :alt: Actuator groups for a legged mobile manipulator
+    :alt: Actuator models for a legged mobile manipulator
+    :width: 80%
+
+.. image:: ../_static/actuator-group/actuator-dark.svg
+    :class: only-dark
+    :align: center
+    :width: 80%
+    :alt: Actuator models for a legged mobile manipulator
 
 .. seealso::
 

docs/source/features/multi_gpu.rst

@@ -27,9 +27,17 @@ Each process collects its own rollouts during the training process and has its o
 network. During training, gradients are aggregated across the processes and broadcasted back to the process
 at the end of the epoch.
 
-.. image:: ../_static/multigpu.png
+.. image:: ../_static/multi-gpu-rl/a3c-light.svg
+    :class: only-light
     :align: center
     :alt: Multi-GPU training paradigm
+    :width: 80%
+
+.. image:: ../_static/multi-gpu-rl/a3c-dark.svg
+    :class: only-dark
+    :align: center
+    :width: 80%
+    :alt: Multi-GPU training paradigm
 
 To train with multiple GPUs, use the following command, where ``--proc_per_node`` represents the number of available GPUs: