Fix Project 2 transparent images

Author: kcpearce

Project#02/README.md

@@ -14,21 +14,33 @@ for the remainder of this project is the water molecule, optimized at the SCF/DZ
 The primary input data for the harmonic vibrational calculation is the Hessian matrix,
 which consists of second derivatives of the energy with respect to atomic positions.
 
-<img src="./figures/hessian.png" height="50">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="./figures/dark/hessian.png">
+  <source media="(prefers-color-scheme: light)" srcset="./figures/hessian.png">
+  <img src="./figures/hessian.png" height="50">
+</picture>
 
 The Hessian matrix (in units of E<sub>h</sub>/a<sub>0</sub><sup>2</sup>) can be downloaded [here](./input/h2o_hessian.txt) for the H<sub>2</sub>O test case. 
 The first integer in the file is the number of atoms (which you should compare to the corresponding value from the geometry file as a test of consistency), 
 while the remaining values have the following format:
 
-<img src="./figures/hessian-file-format.png" width="200">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="./figures/dark/hessian-file-format.png">
+  <source media="(prefers-color-scheme: light)" srcset="./figures/hessian-file-format.png">
+  <img src="./figures/hessian-file-format.png" width="200">
+</picture>
 
  * [Hint 1](./hints/hint1.md): Reading the Hessian
 
 ## Step 3: Mass-Weight the Hessian Matrix
 
 Divide each element of the Hessian matrix by the product of square-roots of the masses of the atoms associated with the given coordinates:
 
-<img src="./figures/mass-weighted-hessian.png" height="50">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="./figures/dark/mass-weighted-hessian.png">
+  <source media="(prefers-color-scheme: light)" srcset="./figures/mass-weighted-hessian.png">
+  <img src="./figures/mass-weighted-hessian.png" height="50">
+</picture>
 
 where m<sub>i</sub> represents the mass of the atom corresponding to atom *i*. Use atomic mass units (amu) for the masses, just as 
 for [Project #1](../Project%2301).
@@ -39,7 +51,11 @@ for [Project #1](../Project%2301).
 
 Compute the eigenvalues of the mass-weighted Hessian:
 
-<img src="./figures/diag-mass-weighted-hessian.png" height="20">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="./figures/dark/diag-mass-weighted-hessian.png">
+  <source media="(prefers-color-scheme: light)" srcset="./figures/diag-mass-weighted-hessian.png">
+  <img src="./figures/diag-mass-weighted-hessian.png" height="20">
+</picture>
 
 You should consider using the same canned diagonalization function 
 you used in [Project #1](../Project%2301).
@@ -50,7 +66,11 @@ you used in [Project #1](../Project%2301).
 
 The vibrational frequencies are proportional to the squareroot of the eigenvalues of the mass-weighted Hessian:
 
-<img src="./figures/vib-freq.png" height="25">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="./figures/dark/vib-freq.png">
+  <source media="(prefers-color-scheme: light)" srcset="./figures/vib-freq.png">
+  <img src="./figures/vib-freq.png" height="25">
+</picture>
 
 The most common units to use for vibrational frequencies is cm<sup>-1</sup>.